Illegal Search and Seizure
The general rule under the Constitution is that a valid warrant is required for a valid search. There are, however, several exceptions to this rule, based on the language of the fourth amendment that "the people" are to be "secure ... against unreasonable searches and seizures".
For instance, the owner of the property in question may consent to the search. The consent must be voluntary, but there is no clear test to determine whether or not it is; rather, a court will consider the "totality of the circumstances" in assessing whether consent was voluntary. Police officers are not required to advise a suspect that he may refuse. There are also some circumstances in which a third party who has equal control, i.e. common authority, over the property may consent to a search.
When an individual does not possess a "reasonable expectation of privacy" that society is willing to acknowledge in a particular piece of property, any interference by the government with regard to that property is not considered a search for Fourth Amendment purposes, and a warrant is never required. For example, courts have found that a person does not possess a reasonable expectation of privacy in information transferred to a third party, such as writing on the outside of an envelope sent through the mail or left for pick-up in an area where others might view it. While that does not mean that the person has no reasonable expectation of privacy in the contents of that envelope, the Court has held that one does not possess a reasonable expectation of privacy that society is willing to acknowledge in the contents of garbage left outside the curtilage of a home. DEA investigators found $5.6 million hidden in a ceiling compartment of a truck during a seizure (Operations Reciprocity, 1997)
There is also a lowered expectation of privacy inside of motor vehicles. This "automobile exception" has been summarized by St. Mary's University law scholar Professor Gerald Reamey in "Reamey's Rule" as "never, ever, ever put anything in your vehicle that you do not want the police to see". Nevertheless, a 'bright line' has been drawn at the doorstep of person's homes, however, so that whenever the government intrudes inside, their action is considered a search for Fourth Amendment purpose and must always be accompanied by a search warrant (absent exigent circumstances).
Courts have also established an "exigent circumstances" exception to the warrant requirement. "Exigent circumstances" simply means that the officers must act quickly. Typically, this is because police have a reasonable belief that evidence is in imminent danger of being removed or destroyed, but there is still a probable cause requirement. Exigent circumstances may also exist where there is a continuing danger, or where officers have a reasonable belief that people in need of assistance are present. This includes when the police are in 'hot pursuit of a fleeing felon.' In this circumstance, so long as there is probable cause, police may follow the suspect into a residence and seize any evidence in plain view.Certain limited searches are also allowed during an investigatory stop or incident to an arrest. These searches are called refined searches.
While the interpretations of the U.S. Supreme Court are binding on all federal courts interpreting the U.S. Constitution, there is some variance in the specifics from state to state, for two reasons. First, if an issue has not been decided by the U.S. Supreme Court, then a lower court makes a ruling of "first impression" on the issue, and sometimes two different lower courts will reach different interpretations. Second, virtually all state constitutions also contain provisions regarding search and seizure. Those provisions cannot reduce the protections offered by the U.S. Constitution, but they can provide additional protections such that a search deemed "reasonable" under the U.S. Constitution might nonetheless be unreasonable under the law of a particular state.
The primary remedy in illegal search cases is known as the "exclusionary rule". This means that any evidence obtained through an illegal search is excluded and cannot be used against the defendant at his or her trial. There are some narrow exceptions to this rule. For instance, if police officers acted in good faith—perhaps pursuant to a warrant that turned out to be invalid, but that the officers had believed valid at the time of the search—evidence may be admitted.