The Fourth Amendment asserts that the "right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated." For more than a century after the founding, interpreting this was relatively straightforward: A "search" was defined along the lines of common law trespass. If police came on to your property to rifle your things, that was a search. If they merely saw you foolishly committing a crime in front of your open window, in "plain view," it was not.
That logic led the Supreme Court to conclude, in the 1928 case Olmstead v. United States, that a wiretap on a suspect's phone wasn't a search, since it required no physical intrusion on the target's property. It was not until Katz v. United States (1957) that the Court changed its mind and ruled that "the Fourth Amendment protects people, not places." New technology that allowed for observation without physical intrusion meant that a "search" would have to be redefined in terms of an "expectation of privacy…that society is prepared to recognize as 'reasonable.'"
In 1983 United States v. Place carved out an important exception to what would count as a "reasonable" expectation of privacy. While the seizure of a cocaine courier's luggage had been unlawful, the court concluded, the use of a drug-sniffing dog to inspect the bags did not count as a search. Without physical intrusion, Justice Sandra Day O'Connor reasoned, a well-trained dog would reveal only whether or not illegal drugs were present—and society was not willing to recognize a privacy interest in illegal drugs.
"In these respects," O'Connor wrote, "the canine sniff is sui generis. We are aware of no other investigative procedure that is so limited both in the manner in which the information is obtained and in the content of the information revealed by the procedure."
It remained unclear just how far the implications of this ruling would extend. The Court seemed to back away from a broad reading of Place when, in Kyllo v. United States (2001), it considered whether a marijuana grower's home had been "searched" under the Fourth Amendment when police, without a warrant, used an infrared imaging device to detect high-intensity plant-growing lamps. Writing for the majority, Justice Antonin Scalia concluded that "obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained" except by physical intrusion "constitutes a search—at least where (as here) the technology in question is not in general public use."
In reply to the argument that the imager had revealed no "intimate details," just blobs of heat, Scalia insisted that in the home "all details are intimate details, because the entire area is held safe from prying government eyes." This standard seemed highly respectful of privacy: If you would have needed a warrant to learn something without some new technology, at least when it came to certain highly protected areas, then you needed a warrant to learn it with technology.
But in early 2005, in Illinois v. Caballes, the court reaffirmed Place, backing off the standard it had seemed to articulate in Kyllo. "Critical to that [Kyllo] decision," Justice John Paul Stevens wrote for the majority (Scalia included), "was the fact that the device was capable of detecting lawful activity," such as when occupants might be using the bath or sauna. So Caballes established an exception to Kyllo's relatively protective rule for technologically enhanced searches: A technique that could, with reasonable accuracy, discover contraband without exposing intimate information would not count as a search, though it could establish probable cause for a subsequent full-blown search.