From Heath's article:
At least 50 U.S. law enforcement agencies have secretly equipped their officers with radar devices that allow them to effectively peer through the walls of houses to see whether anyone is inside, a practice raising new concerns about the extent of government surveillance.
. . .In Denson, the Tenth Circuit held that officers' entry and search of a home was constitutional. In that case, officers used a radar device to determine whether anybody was present inside the home. The court declined to rule on the constitutionality of this device, but noted that significant Fourth Amendment problems would have arisen if the evidence the radar device revealed had been crucial to the government's case. From the opinion:
The radars work like finely tuned motion detectors, using radio waves to zero in on movements as slight as human breathing from a distance of more than 50 feet. They can detect whether anyone is inside of a house, where they are and whether they are moving.
Separately and as we alluded to earlier, the government brought with it a Doppler radar device capable of detecting from outside the home the presence of “human breathing and movement within.” All this packed into a hand-held unit “about 10 inches by 4 inches wide, 10 inches long.” The government admits that it used the radar before entering — and that the device registered someone’s presence inside. It’s obvious to us and everyone else in this case that the government’s warrantless use of such a powerful tool to search inside homes poses grave Fourth Amendment questions. New technologies bring with them not only new opportunities for law enforcement to catch criminals but also new risks for abuse and new ways to invade constitutional rights. See, e.g., Kyllo v. United States, 533 U.S. 27, 33-35 (2001) (holding that using warrantless thermal imaging to show activity inside a home violated the Fourth Amendment). Unlawful searches can give rise not only to civil claims but may require the suppression of evidence in criminal proceedings. We have little doubt that the radar device deployed here will soon generate many questions for this court and others along both of these axes. At the same time, in a criminal proceeding like ours the government is free to rely on facts gleaned independently from any Fourth Amendment violation. See Murray v. United States, 487 U.S. 533, 537 (1988). And in our case Mr. Denson acknowledges that all of the facts we’ve outlined above were discovered independently of the potentially problematic radar search — a fact that requires us to defer those questions to another day.Under Kyllo v. United States, law enforcement officers undertake a Fourth Amendment search when they use "a device that is not in general public use, to explore details of the home that would previously have been unknowable without physical intrusion" and this search is "presumptively unreasonable without a warrant."
Kyllo may no longer be good law with regard to infrared scanners because this technology is becoming increasingly popular and therefore more likely to be "in general public use." FLIR, the company that manufactured the camera at issue in Kyllo, is now marketing an infrared camera that people can attach to their iPhones.
But even if infrared cameras are so popular that their use is no longer considered a Fourth Amendment search, police radars may fall under Kyllo's prohibition. Radars used to detect human movement or breathing are most likely not in general public use. So even if Kyllo is no longer good law with regard to the infrared technology it discussed, its rule may live on to apply to new forms of technology. Police radars may end up being one such form of technology.