Some 50 police agencies including U.S. marshals and the FBI have been using for two years Range-R doppler radar devices that can see 50 feet through walls, including brick and concrete ones, to detect the location of people inside their houses. And in some cases law enforcement officers are using with them without search warrants. As USA Today reports:

The radars were first designed for use in Iraq and Afghanistan. They represent the latest example of battlefield technology finding its way home to civilian policing and bringing complex legal questions with it…. 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.

Certainly, such information could be vital to the protection of police officers who are executing valid search warrants based on probable cause. The problem is that law enforcement has not always sought a warrant before deploying the technology. The USA Today article cites the case of fugitive parolee Steven Denson in which the Marshals Service used the radar device to detect his presence in a house in Wichita, Kansas.

The marshals had an arrest warrant for Denson, but not a search warrant for that location. Interestingly, the marshal's report did not mention the use of the radar; merely that they "had developed a reasonable suspicion" that Denson was in the house. An interesting omission. Denson challenged his arrest arguing that marshal's would not have had a reasonable belief that he was in the house without the use of radar device.

The Tenth Circuit of the U.S. Appeals Court disagreed, noting that the marshals had sufficient independent evidence of his presence, e.g., Denson had opened an electric utility account for the house, a whirring electric meter outside, and the fact that an unemployed Denson was likely to be home at 8:30 a.m. On that basis, the Appeals Court upheld his arrest. The Appeals Court did note…

…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. … 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.

The Appeals Court specifically cited the 2001 U.S. Supreme Court decision in the case of Kyllo v. United States in which federal agents scanned a house with a thermal imager with the goal of detecting heat emitted from grow lights used in marijuana cultivation. Based on the imager's finding that a lot of heat was emanating from the suspect's garage, the agents obtained a search warrant and raided the house. The U.S. Supreme Court remanded the case after holding:

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.

On the basis of that standard, if the only clue that Denson was at home were the result of the radar scan, then he presumably would have had a case against the Marshal Service's intrusion into his residence.

In any case, Range-R is certainly not the last word in radar surveillance. USA Today notes:

Other radar devices have far more advanced capabilities, including three-dimensional displays of where people are located inside a building, according to marketing materials from their manufacturers. One is capable of being mounted on a drone. And the Justice Department has funded research to develop systems that can map the interiors of buildings and locate the people within them.

With regard to a person's reasonable expectation of privacy, the majority in Kyllo noted:

…in the case of the search of the interior of homes–the prototypical and hence most commonly litigated area of protected privacy–there is a ready criterion, with roots deep in the common law, of the minimal expectation of privacy that exists, and that is acknowledged to be reasonable. To withdraw protection of this minimum expectation would be to permit police technology to erode the privacy guaranteed by the Fourth Amendment. We think that obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained without physical "intrusion into a constitutionally protected area," Silverman, 365 U.S., at 512, constitutes a search–at least where (as here) the technology in question is not in general public use. This assures preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted. On the basis of this criterion, the information obtained by the thermal imager in this case was the product of a search.

The "not in general public use" standard seems awfully permeable on its face. One dystopian interpretation is that the more widely surveillance technologies are deployed the less reasonable our expectations of privacy will become.