Law enforcement officers quickly get used to constantly answering the question, "Why did you do that, officer?" In your search warrant affidavits, your reports, and your testimony you have to lay out the basis of your suspicions and justify every detention, arrest, search, seizure, entry, and use of force.
Otherwise, the warrant doesn't get issued, your case gets rejected by supervisors or prosecutors, a judge orders the suppression of evidence, or you find yourself facing a civil rights lawsuit. Your world is full of requirements that you articulate reasonable suspicion, probable cause, or other level of justification for what you've done.
Although you already know there's no necessity to explain why you search your jail cells or examine abandoned property or seize contraband or evidence in plain view, you might be surprised to learn that there are some other things you don't have to justify.
The U.S. Supreme Court has said that not every interaction between police and citizens amounts to a search or seizure requiring some kind of suspicion. If you simply walk up to an individual on the street and start talking, without blocking the way or ordering the person to stay and talk to you, the person has not been detained and you have nothing to justify. Said the court, "There is nothing in the Constitution which prevents a policeman from addressing questions to anyone on the streets." (U.S. v. Mendenhall)
Assuming either a consensual encounter or a justified detention or arrest, you do not have to justify asking a person investigative questions. For example, where police questioned Iris Mena during service of a search warrant, the Supreme Court ruled that officers need not justify such questioning: "Even when officers have no basis for suspecting an individual, they may generally ask questions. The officers did not need reasonable suspicion to ask Mena for her name, date and place of birth, or immigration status." (Muehler v. Mena)
Marked police cars and motorcycles are allowed to drive along public streets and highways, the same as other vehicles. As long as no colored lights or sirens are used and the police car doesn't cut off another car or block a pedestrian's route, the fact that officers show an interest in someone or drive behind or parallel to someone does not infringe the person's liberty or require explanation.
In Michigan v. Chesternut, the suspect began running away when a marked car approached the street corner where he was selling drugs. The officers turned the corner and followed, saw him throw down drugs, and subsequently stopped and arrested Chesternut. Although the state court thought Chesternut had been detained by the officers' conduct in following him, the U.S. Supreme Court disagreed and reversed the suppression order.
In the court's words, "While the very presence of a police car driving parallel to a running pedestrian could be somewhat intimidating," it does not constitute a seizure or necessitate any articulated suspicion.
Surveillance Driving and Beepers
Undercover surveillance driving runs the risk of detection by the suspects. Attaching an electronic transponder ("beeper") to the undercarriage of the suspect vehicle (or planting it in precursor chemicals or bait money, for example) allows officers to follow from a distance and identify the suspects' ultimate location, in order to obtain a search warrant. Non-intrusive installation of a beeper and monitoring it over public highways do not require a warrant or suspicion to satisfy the Fourth Amendment because "A person traveling on public thoroughfares has no reasonable expectation of privacy in his movements from one place to another." (U.S. v. Knotts)
Flashlights, Spotlights, and Binoculars
What can lawfully be seen with the naked eye in daylight can also be seen during darkness by use of flashlights and spotlights: "The use of artificial means to illuminate a darkened area simply does not constitute a search, and thus triggers no Fourth Amendment protection." (Texas v. Brown)
What can lawfully be watched from nearby can lawfully be watched through telescopes or binoculars from a distance: "The use of field glasses or the telescope to magnify the object of a witness' vision is not a forbidden search or seizure." (On Lee v. U.S.)
License Plates and VINs
Both a license plate ("tag") and an embossed vehicle identification number (after 1969) are required to be visible from the exterior. When you note a plate number and run it through NCIC or other databases, you are not engaging in a search, so no suspicion is needed. Likewise, when you approach a parked or lawfully stopped vehicle and examine the VIN, no reasons need be given:
"It is unreasonable to have an expectation of privacy in an object required by law to be located in a place ordinarily in plain view from the exterior of the automobile. The exterior of a car, of course, is thrust into the public eye, and thus to examine it does not constitute a 'search'." (New York v. Class)
Vehicles cannot be randomly stopped just to see if a trained dog "hits" on them for drugs or explosives. (Indianapolis v. Edmond) However, if the car is parked or is stopped during a lawful detention, running the dog around the exterior to see if it reacts is not a search, and needs no independent suspicion:
"A dog sniff conducted during a concededly lawful traffic stop that reveals no information other than the location of a substance that no individual has any right to possess does not violate the Fourth Amendment." (Illinois v. Caballes)
Ordering Drivers and Passengers Out
One study indicated that 30 percent of officers shot in the line of duty were shot as they approached someone seated in a vehicle. The Supreme Court has determined that this risk justifies a flat rule that at any lawful traffic stop, both the driver and all passengers may be ordered out, in the officer's discretion, as a routine safety precaution, no explanation required.
"We hold that once a motor vehicle has been lawfully detained for a traffic violation, the police officers may order the driver to get out of the vehicle." (Pennsylvania v. Mimms) "We hold that an officer making a traffic stop may order passengers to get out of the car pending completion of the stop." (Maryland v. Wilson)
And in Brendlin v. California, the Supreme Court said that passengers should not consider themselves free to depart without police permission. Therefore, their temporary detention would require no additional justification beyond the reasonable suspicion for the initial traffic stop.
Requesting Consent to Search
You do not have to suspect that a search will reveal something incriminating before requesting consent. During a consensual encounter (U.S. v. Drayton) or following a lawful detention (Florida v. Jimeno) or arrest (U.S. v. Watson), you may simply ask for consent to search, no explanation required: "Even when officers have no basis for suspecting an individual, they may generally request consent to search." (Florida v. Bostick)
A few states place greater restrictions on their officers than exist under the Constitution, as interpreted in these U.S. Supreme Court decisions. Always consult local advisors to verify the rules for your jurisdiction.