The Fourth Amendment protects against "unreasonable searches and seizures," but there's been debate over what constitutes a "search."
One of the earliest cases dealing with the definition of a police search comes from Olmstead v. U.S. The question in this case was whether a wiretap counted as a search, which would require the police to procure a warrant with probable cause. The Supreme Court ruled that wiretaps didn't count as a search under the Fourth Amendment. A search, according to the Supreme Court, was defined as an act of "physical penetration" used to procure information. In the case of Olmstead v. U.S., there was no "physical penetration" into one's "house" or "effects."
However, the Supreme Court dramatically reversed its opinion from Olmstead v. U.S. in the 1967 case, Katz v. U.S.
The Warren Court ruled that wiretaps did constitute as a search under the Fourth Amendment, because the law protects "people, not places." Thus, the words of an individual would also protected from intrusion. The Katz test was formulated to determine if a warrant would be needed to conduct a search. The first part examined whether the person in the situation would have a "reasonable expectation of privacy." In the case of Katz, he was in a phone booth and would have "reasonably expected" that he would have some form of privacy during his phone conversation. The second part of the test determined whether society would accept the privacy expectation outlined in the first part. The court reasoned that society would generally accept that Katz would have privacy while using the phone booth.
Using the Katz test, the Supreme Court has taken on multiple cases, which helped clarify exactly what authorized a search without a warrant.
One example is whether trash left on a curb could be searched without a warrant. The court ruled in California v. Greenwood that trash left on a curb doesn't have a reasonable expectation of privacy because creatures, kids and others could look into the trash. In Oliver v. United States, the court upheld the open fields doctrine that was established in Hester v. U.S., which permitted warrantless searches of open fields. The Supreme Court reasoned that there's no reasonable expectation of privacy for open fields since anyone can stumble upon the property.
In modern times, court cases focus on the legality of searches via aerial surveillance.
In 1989, the Supreme Court ruled in Florida v. Riley that it was permissible to use a police helicopter 400 feet in the air to observe land without a warrant, because private individuals helicopters could also legally fly with a private helicopter. Florida v. Riley has set the stage for future cases regarding drones, which has yet to reach the Supreme Court. Now faced with new technological advances such as drones, the Supreme Court will be busy deciding if the usage of surveillance drones constitute as a search.
