JOHNSON, Circuit Judge:
This case arises from the district court’s denial of co-defendants Ronald Tobin’s and Clifford Aekerson’s motions to suppress evidence seized in a search of Ackerson’s home, garage, and a station wagon parked in the garage. A panel of this Court reversed the district court’s finding that the search was permissible under the Fourth Amendment and held that Ackerson’s motion should have been granted. The panel, however, affirmed the district court’s denial of Tobin’s motion, holding that Tobin did not have standing to contest the legality of the search. We vacated the panel’s opinion and now affirm en banc the district court’s denial of both motions to suppress.
I. STATEMENT OF THE CASE
A. Factual Background
The facts of this case are extensively reported in the panel’s opinion. United States v. Tobin, 890 F.2d 319 (11th Cir.1989), reh’g granted and opinion vacated, [1508]*1508902 F.2d 821 (11th Cir.1990). We therefore give only the following brief summary.
On March 19, 1986, two United States Customs agents and two Drug Enforcement Administration agents were conducting a surveillance unrelated to the instant case in a residential neighborhood in Miami. The agents watched the neighborhood from a field located northwest of the front of defendant Clifford Ackerson’s house. Around 3:30 in the afternoon, one of the agents noticed a Mercury Marquis abruptly stop in front of Ackerson’s house and back into the driveway. The agents began watching the activity through binoculars. Defendant Ronald Tobin got out of the car, looked around, ran to the front door, knocked, and was admitted within one minute. Shortly thereafter, Ackerson lifted the garage door three-quarters of the way up, looking up and down the street as he held the door. Tobin emerged and unlocked the trunk of the Mercury. He removed three clear plastic tubular bags, each approximately four feet long. According to one agent’s testimony, smaller bundles were visible inside the bags. To-bin quickly put the bags in the garage. While the garage door was open, the agents observed a station wagon parked inside. A third vehicle, a pickup truck, was parked in the front yard.
Believing that the tubular bags contained cocaine, three of the agents decided to go to the door and talk to the occupants of the house. The fourth agent remained at the surveillance post. The three agents pulled up in front of the house in their individual unmarked vehicles. One went to the door; another accompanied the first as security backup; the third went to the corner of the house near the garage to stand out of sight.
The first agent knocked and received no answer. He continued knocking for three to four minutes, calling out in English and Spanish.1 Ackerson opened the door. The agent showed him his credentials, told him he wanted to talk to him about the car in the driveway, and explained that he believed someone had put cocaine in his garage. Ackerson denied that anyone had recently backed into the driveway and stated that no one else was in his home. The agent smelled marijuana coming from inside the house as they spoke. Ackerson eventually called Tobin to the door. Tobin denied having driven the car there. When the agent explained all he had observed, Tobin continued to deny it.
The agent then told the two men that he thought they should all go to the garage and see what was there. Ackerson turned and walked inside the house in the direction of the garage. The agent followed him into the house. Tobin and the backup agent followed to the rear. When they got to the garage, the first agent asked Acker-son to open the outside garage door. He moved around the station wagon and opened it. The third agent, standing outside on the driveway, looked into the garage and spotted the tubular bags on the floor. One of the bags had been opened. The agent could see that it contained cocaine. The agents then placed Ackerson and Tobin under arrest. The agents did a security sweep of the house and discovered three bales of marijuana in the shower stall of the bathroom connected to the master bedroom. One of the agents also noticed through the windows of the station wagon in the garage that the screws had been removed from the floorplate over the wheel well in the rear of the vehicle. Upon opening the back of the vehicle and lifting up the wheel well cover, the agent found grocery bags full of cash, totalling $775,000.
B. Procedural History
1. The District Court
Ackerson and Tobin were charged with conspiracy and possession with intent to distribute cocaine in violation of 21 U.S. C.A. §§ 841(a)(1) & 846 and 18 U.S.C.A. § 2. Ackerson alone was charged with possession with intent to distribute marijuana in violation of 21 U.S.C.A. § 841(a)(1).
Both defendants filed motions to suppress the fruits of the searches. After [1509]*1509holding a hearing on the motions, the magistrate recommended that the district court grant both motions to suppress, finding that the searches were illegal and that To-bin as well as Ackerson had standing to contest the searches.
After the government objected to the magistrate’s report, the district court held a de novo hearing on the standing issue. The court subsequently entered an order denying both motions to suppress, holding that, based on the magistrate’s findings, the search of the garage and protective sweep of the house were legal and that neither defendant had standing to contest the search of the station wagon because both had denied ownership of it. The court was thus not required to address Tobin’s standing as to the house and garage, though it commented that the evidence adduced at the hearing did not support a finding that Tobin had a privacy interest in the house or garage to which Fourth Amendment protections could attach.
Tobin and Ackerson thereafter entered plea agreements with the government whereby they would both plead guilty to possession with intent to distribute cocaine, reserving the right to appeal the denial of their motions to suppress and to withdraw the guilty pleas if they prevailed. They subsequently pled guilty pursuant to the agreement. The court sentenced both defendants to terms of fifteen years in prison and fines of $100,000. Both defendants remained free on bond pending the appeal.
2. The Panel
The panel reversed the district court’s denial of Ackerson’s motion to suppress, but affirmed the denial of Tobin’s motion. Tobin, 890 F.2d at 382. In the panel’s view, the search of the house violated the Fourth Amendment. The panel noted that the magistrate and district court had found that the agents had reasonable suspicion of criminal activity when they approached the house.2 This suspicion alone did not, in the panel’s opinion, justify detaining and questioning the occupants under Terry v. Ohio, 392 U.S. 1, 88 S.Ct. 1868, 20 L.Ed.2d 889 (1968), as the district court had opined, because Terry does not apply to dwellings. The key issue then, according to the panel, was whether the agent’s conduct at the door of the house, which afforded him the opportunity to smell the marijuana, precipitated an impermissible search to which Ackerson did not consent.
The panel stated that a police officer’s approach to a criminal suspect’s house to make general inquiries is not a search as long as the door is not opened in response to a threat or command. United States v. Knight, 451 F.2d 275, 278 (5th Cir.1971), cert. denied, 405 U.S. 965, 92 S.Ct. 1171, 31 L.Ed.2d 240 (1972). Because in its view the agent’s conduct at the door amounted to a demand of entry under color of authority, however, the panel found the opening of the door to be a search to which Ackerson could not have consented. The olfactory access to the house which the agent gained pursuant to the opening of the door was therefore illegal. The searches which followed were consequently tainted. The panel also found, however, that Tobin did not have standing to assert the Fourth Amendment violation. Consequently, the evidence uncovered in the search was admissible against him.
3. Rehearing En Banc
Tobin then filed a petition for a rehearing and a suggestion for rehearing en banc on the standing issue. The government did not file a cross-petition. Ackerson, who had already obtained complete relief, did not join in Tobin’s petition. On May 16, 1990, we vacated the entire panel opinion and granted rehearing en banc. At en banc oral argument, the parties argued both the issue of standing and the issue of the search’s legality. It is not necessary to the outcome of this case, however, that we reach the standing issue. For the purposes of the following discussion, we assume, but do not hold, that the parties have standing to raise the question of the search’s legality.
[1510]*1510II. ANALYSIS
Review of a district court's denial of a motion to suppress evidence is a mixed question of law and fact. United States v. Alexander, 835 F.2d 1406, 1408 (11th Cir.1988). The district court’s findings of fact are reviewed under the clearly erroneous standard, whereas its application of the law to those facts is subject to de novo review. Id, In reviewing the district court’s ruling, this Court must construe the facts in the light most favorable to the party prevailing below, i.e., here the government. United States v. Wilson, 894 F.2d 1245, 1254 (11th Cir.), cert. denied, — U.S.-, 110 S.Ct. 3284, 111 L.Ed.2d 792 (1990).
The warrantless search of a home is “presumptively unreasonable.” Payton v. New York, 445 U.S. 573, 586, 100 S.Ct. 1371, 1380, 63 L.Ed.2d 639 (1980). A war-rantless search is allowed, however, where both probable cause and exigent circumstances exist. United States v. Burgos, 720 F.2d 1520, 1525 (11th Cir.1983).
A. The Agents’ Approach to the House 1. Probable Cause
The magistrate and district court found that the agents had reasonable suspicion of criminal activity at the time they made their observations through binoculars from the field. Probable cause, in the district court’s view, did not arise until the agent, while standing at the door, smelled the marijuana. The government did not contest this finding.
The question of what amounts to “probable cause is purely a question of law and hence is subject to plenary review by this court.” United States v. Hurtado, 779 F.2d 1467, 1477 (11th Cir.1985). Probable cause exists when under the “totality-of-the-circumstances ... there is a fair probability that contraband or evidence of a crime will be found in a particular place.” Illinois v. Gates, 462 U.S. 213, 238, 103 S.Ct. 2317, 2332, 76 L.Ed.2d 527 (1983). In other words, probable cause exists “where the facts lead a reasonably cautious person to believe that the ‘search will uncover evidence of a crime.’ ” United States v. Burgos, 720 F.2d at 1525 (quoting United States v. Rojas, 671 F.2d 159, 165 (5th Cir. Unit B 1982)).
Prior to their approach to the house, the agents observed the defendants behave suspiciously, look about furtively, and quickly transfer into the garage tubular bags which contained smaller bundles. In view of these facts, the agents could reasonably have concluded that a search would uncover evidence of a crime. Because there was a “fair probability” that cocaine would be found in the garage, it appears that the agents had probable cause to search even prior to their approach to the house.
2. Exigent Circumstances
To justify a warrantless search, however, there must also exist exigent circumstances in addition to probable cause. Burgos, 720 F.2d at 1525. This Circuit has held that the “presence of contraband without more does not give rise to exigent circumstances.” United States v. Torres, 705 F.2d 1287, 1297 (11th Cir.), vacated and remanded on other grounds, 718 F.2d 998 (11th Cir.1983). An exigent situation may arise, however, when there is danger that the evidence will be destroyed or removed. Burgos, 720 F.2d at 1526. See also United States v. Standridge, 810 F.2d 1034, 1037 (11th Cir.) (listing several factors indicating existence of exigent circumstances), cert. denied, 481 U.S. 1072, 107 S.Ct. 2468, 95 L.Ed.2d 877 (1987). This Court has held that the need to invoke the exigent circumstances exception to the warrant requirement is “particularly compelling in narcotics cases” because narcotics can be so quickly destroyed. United States v. Young, 909 F.2d 442, 446 (11th Cir.1990). The test of whether exigent circumstances exist is an objective one. Id. “ ‘[T]he appropriate inquiry is whether the facts ... would lead a reasonable, experienced agent to believe that evidence might be destroyed before a warrant could be secured.’ ” Id. (quoting United States v. Rivera, 825 F.2d 152, 156 (7th Cir.), cert. denied, 484 U.S. 979, 108 S.Ct. 494, 98 L.Ed.2d 492 (1987)).
[1511]*1511Prior to their approach to the house, the agents had probable cause to believe that narcotics were in the garage or the house and knew that at least two persons were in the house. They also knew that there were three vehicles on the premises, a fact which raises the possibility that there were more than two persons inside the house. The presence of the vehicles also permits the inference that the suspects might depart with or without the narcotics even before the agents could reach the house.
Circumstances are not normally considered exigent where the suspects are unaware of police surveillance. Torres, 705 F.2d at 1297; United States v. Cravero, 545 F.2d 406, 414-15 n. 24 (5th Cir.1976), cert. denied, 430 U.S. 983, 97 S.Ct. 1679, 52 L.Ed.2d 377 (1977). This Circuit has held, moreover, that a warrantless search is illegal when police possess probable cause but instead of obtaining a warrant create exigent circumstances. See United States v. Scheffer, 463 F.2d 567, 575 (5th Cir.), cert. denied, 409 U.S. 984, 93 S.Ct. 324, 34 L.Ed.2d 248 (1972); see also United States v. Munoz-Guerra, 788 F.2d 295, 298 (5th Cir.1986) (holding that where agents can get a warrant instead of revealing themselves and making immediate entry a foregone necessity, a warrantless search must be deemed unreasonable); but see United States v. MacDonald, 916 F.2d 766 (2d Cir.1990) (in holding a warrantless search valid where police had probable cause but precipitated the exigent circumstances by approaching the dwelling and knocking, the court stated, “The fact that the suspects may reasonably be expected to behave illegally does not prevent law enforcement agents from acting lawfully to afford the suspects the opportunity to do so. Thus, assuming arguendo that there were no exigent circumstances before the knock, the agents’ conduct did not impermissibly create the circumstances occurring thereafter.”). It should be noted, on the other hand, that “[l]aw enforcement officers are under no constitutional duty to call a halt to a criminal investigation the moment they have the minimum evidence to establish probable cause, a quantum of evidence which may fall far short of the amount necessary to support a criminal conviction.” Hoffa v. United States, 385 U.S. 293, 310, 87 S.Ct. 408, 417, 17 L.Ed.2d 374 (1966).
In the instant case, the agents could reasonably conclude from the defendants’ hurried actions and furtive looks that the defendants were either aware or afraid that someone was watching them.3 Destruction or removal of some portion of the narcotics was therefore a possibility. Construing the facts in a light most favorable to the government, the prevailing party below, it therefore appears that there existed exigent circumstances which would permit a warrantless search even prior to the approach to the house.
B. Approach to the House
Assuming arguendo that the agents’ observations considered in the totality of the circumstances did not immediately give rise to the necessary level of probable cause and exigent circumstances, the warrantless search should still, be deemed justifiable. The district court, the panel, and all parties agreed that based upon the binocular observations, the agents had, at a minimum, reasonable suspicion that criminal activity was afoot. Reasonable suspicion cannot justify the warrant-less search of a house, see Arizona v. Hicks, 480 U.S. 321, 328, 107 S.Ct. 1149, 1154, 94 L.Ed.2d 347 (1987), but it can justify the agents’ approaching the house to question the occupants. Knight, 451 F.2d at 278; Davis v. United States, 327 F.2d 301, 303 (9th Cir.1964) (stating “[ajbsent express orders from the person in possession against any possible trespass, there is no rule of private or public conduct which makes it illegal per se, or a condemned invasion of the person’s right of privacy, for anyone openly and peaceably, at high noon, to walk up the steps and knock on the front door of any man’s ‘castle’ with the honest intent of asking questions of the occupant thereof — whether the questioner be a pollster, a salesman, or an officer of the law.”).
[1512]*1512There is no doubt that the agent’s suspicions rose to the level of probable cause when, as the door stood open, he detected what he knew from his law enforcement experience to be the odor of marijuana.4 United States v. Lueck, 678 F.2d 895, 903 (11th Cir.1982). Moreover, the defendants and anyone else who might have been present in the house would have been aware of the agent’s suspicions at that moment. Danger that the defendants or someone else inside the house might destroy the evidence thus provided the exigent circumstances required to justify a warrantless search. Burgos, 720 F.2d at 1526. Consequently, if the agents possessed only reasonable suspicion at the time of their observations and approached the house legally under Knight to speak with its occupants in order to dispel the suspicion, the search can be found illegal only if Ackerson’s opening of the door was not consensual. United States v. Edmondson, 791 F.2d 1512, 1514 (11th Cir.1986).5
The voluntariness of consent must be judged in light of the totality of the circumstances. Schneckloth v. Bustamonte, 412 U.S. 218, 227, 93 S.Ct. 2041, 2047-48, 36 L.Ed.2d 854 (1973). If the circumstances indicate that Ackerson opened the door in response to a “show of official authority,” then Ackerson cannot be deemed to have consented to the agent’s obtaining the olfactory evidence indicating the presence of marijuana. Edmondson, 791 F.2d at 1515. The relevant facts appear to be that the agent, with a backup officer standing behind him, knocked continuously for three to four minutes, calling out to the occupants. The agents were in plain clothes. Though they were armed, they did not display their weapons.
Whether the consent is deemed involuntary depends upon the amount of threat presented. On the one hand, cases in which police have used their position to demand entry have held that consent was not voluntary and thus have required suppression of evidence discovered pursuant to entry. Bumper v. North Carolina, 391 U.S. 543, 548, 88 S.Ct. 1788, 1791-92, 20 L.Ed.2d 797 (1968) (holding that entry and search was not consensual when consent was given only after police stated that they possessed a warrant); Edmondson, 791 F.2d at 1515 (holding that entry was not consensual when consent was given only after agent with weapon drawn shouted, “FBI. Open up.”). On the other hand, consent to enter has been held voluntary where the agent first telephoned and requested an interview, and the defendant agreed, subsequently opening the door when the agent knocked and identified himself by showing his credentials. United States v. Willis, 759 F.2d 1486, 1493 & 1498 (11th Cir.), cert. denied, 474 U.S. 849, 106 S.Ct. 144, 88 L.Ed.2d 119 (1985).
The facts of the instant case fall much closer to those of Willis than those of Bumper or Edmondson. In calling out to the occupants of the house, the agent did not use the imperative as did the officer in Edmondson. On the contrary, he phrased his words in the form of a request. The occupants were free to deny that request or alternatively talk to the agent through the closed door. The decision to open the door was therefore voluntary. Consequently, Ackerson may be deemed to have consented to the agent’s smelling of the marijuana which gave rise to the probable cause needed to do a warrantless search if that probable cause did not previously exist.6
[1513]*1513C. Searches Subsequent to Entry
Because we find that warrant-less entry into Ackerson’s house justifiable, we hold the search of the garage and seizure of the cocaine to be permissible under the Fourth Amendment. Moreover, the protective sweep of the house subsequent to the arrests was also permissible. “The Fourth Amendment permits a properly limited protective sweep in conjunction with an in-home arrest when the searching officer possesses a reasonable belief based on specific and articulable facts that the area to be swept harbors an individual posing a danger to those on the arrest scene.” Maryland v. Buie, — U.S. -, 110 S.Ct. 1093, 1099-1100, 108 L.Ed.2d 276 (1990). The fact that there were three vehicles on the scene coupled with Ackerson’s lying about Tobin’s presence clearly gave rise to a reasonable belief that someone else could be hiding in the house. The agents were, of course, free to seize any evidence they discovered in plain view within the proper scope of the protective sweep. Coolidge v. New Hampshire, 403 U.S. 443, 465, 91 S.Ct. 2022, 2037, 29 L.Ed.2d 564 (1971) (“Where the initial intrusion that brings the
police within plain view of [contraband] is supported, not by a warrant, but by one of the recognized exceptions to the warrant requirement, the seizure is ... legitimate.”). Discovery and seizure of the marijuana in the shower stall was therefore legitimate.7 Moreover, the search of the station wagon may be justified on the basis of the automobile exception. United States v. Ross, 456 U.S. 798, 807-808, 102 S.Ct. 2157, 2163-2164, 72 L.Ed.2d 572 (1982). The agent’s sighting of the screws removed from the wheel well cover clearly gave rise to probable cause to believe the wheel well contained contraband. The evidence of exigent circumstances need not be overwhelming to justify the warrantless search of an automobile. Alexander, 835 F.2d at 1409. Both Tobin and Ackerson denied ownership of the vehicle, indicating to the agent that the true owner could have arrived and driven the vehicle away, thus depriving the government of evidence if the agent did not act immediately. See id.
III. CONCLUSION
For the foregoing reasons, we AFFIRM the district court’s denial of Ackerson’s and [1514]*1514Tobin's motions to suppress the evidence found pursuant to the search of the garage, house, and station wagon.