Kyllo v. United States

Kyllo v. United States, 533 U.S. 27 (2001), was an unusual 5-4 decision led by Justice Antonin Scalia and joined by conservative Justice Clarence Thomas along with three liberal Justices, in order to invalidate the use of an heat sensor outside of one's home to detect lamps used to grow illegal marijuana inside the home. A prior warrant had not been obtained for what the Court considered to be a potential invasion of privacy. The rationale was that this heat sensor could detect (and thus constitute a privacy invasion) of lawful activity as well as unlawful activity.

This was a rare case in which Justice Anthony Kennedy and John Paul Stevens were on the conservative side but Justices Scalia and Thomas were on the liberal side.

Where only unlawful activity can be detected, such as the use of drug-sniffing dogs around the trunk of a car stopped for speeding, the U.S. Supreme Court upheld the search without requiring a warrant in Illinois v. Caballes, 533 U.S. 27 (2005).