KYLLO v. UNITED STATES 533 U.S. 27
Held: Where, as here, the Government uses a device that is not in general public use, to explore details of a private home that would previously have been unknowable without physical intrusion, the surveillance is a Fourth Amendment "search," and is presumptively unreasonable without a warrant. Pp. 31-41.
(a) The question whether a warrantless search of a home is reasonable and hence constitutional must be answered no in most instances, but the antecedent question whether a Fourth Amendment "search" has occurred is not so simple.
This Court has approved warrantless visual surveillance of a home, see California v. Ciraolo, 476 U. S. 207, 213, ruling that visual observation is no "search" at all, see Dow Chemical Co. v. United States, 476 U. S. 227, 234-235, 239. In assessing when a search is not a search, the Court has adapted a principle first enunciated in Katz v. United States, 389 U. S. 347, 361:
A "search" does not occur-even when its object is a house explicitly protected by the Fourth Amendment-unless the individual manifested a subjective expectation of privacy in the searched object, and society is willing to recognize that expectation as reasonable, see, e. g., California v. Ciraolo, supra, at 211. Pp. 31-33.
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One might think that the new validating rationale would be that examining the portion of a house that is in plain public view, while it is a "search" 1 despite the absence of trespass, is not an "unreasonable" one under the Fourth Amendment. See Minnesota v. Carter, 525 U. S. 83, 104 (1998) (BREYER, J., concurring in judgment).
But in fact we have held that visual observation is no "search" at all perhaps in order to preserve somewhat more intact our doctrine that warrantless searches are presumptively unconstitutional. See Dow Chemical Co. v. United States, 476 U. S. 227, 234-235, 239 (1986).
In assessing when a search is not a search, we have applied somewhat in reverse the principle first enunciated in Katz v. United States, 389 U. S. 347 (1967). Katz involved eavesdropping by means of an electronic listening device placed on the outside of a telephone booth-a location not within the catalog ("persons, houses, papers, and effects") that the Fourth Amendment protects against unreasonable searches. We held that the Fourth Amendment nonetheless protected Katz from the warrantless eavesdropping because he "justifiably relied" upon the privacy of the telephone booth. Id., at 353.
As Justice Harlan's oft-quoted concurrence described it, a Fourth Amendment search occurs when the government violates a subjective expectation of privacy that society recognizes as reasonable. See id., at 361