Final answer:
The Fourth Amendment requires a search warrant based on probable cause for searches and seizures, but there are exemptions when a person lacks 'reasonable expectation of privacy' or under exigent circumstances. Consent, witnessing a crime, and certain scenarios like car searches do not always require a warrant. The warrant requirement has been both criticized and defended for its flexibility in application.
Step-by-step explanation:
Persons present at the scene of a search warrant execution can indeed be detained during the search. The Fourth Amendment protects against unreasonable searches and seizures, requiring that a warrant must be secured upon probable cause and must describe the place to be searched, and the persons or things to be seized. However, the Supreme Court has recognized exceptions where a person lacks a "reasonable expectation of privacy" or if exigent circumstances exist. Such circumstances include situations where there is a risk of evidence being tampered with or destroyed, or if items are in plain view. Additionally, consent from the owner or renter can also negate the need for a warrant.
Furthermore, police can arrest individuals without a warrant if there is probable cause to suspect they have committed a felony, or if they witness a person committing a misdemeanor. The courts have also granted leeway in situations like the search of a passenger compartment of a car or the search of people entering the U.S., where a warrant is not generally required. Nonetheless, critics argue that the system favors law enforcement as they can usually obtain a warrant fairly easily, though some have argued that police are unlikely to waste resources on warrants that are unlikely to be granted. Recent events have led to a scrutiny and reevaluation of policies like the use of "no-knock" warrants.