The use of technology that allows the police to "see" inside the homes of suspects has raised privacy questions.
At least 50 US police forces are believed to be equipped with radars that can send signals through walls.
The use of the radar device, known as Range-R, was made public in a Denver court late last year.
It was used by police entering a house to arrest a man who had violated the terms of his parole.
In court documents relating to the case, lawyers defending Steven Denson questioned whether officers entered his home lawfully.
One of the questions asked was "how the Fourth Amendment interacts with the government's use of radar technology to peer inside a suspect's home".
Although the judges upheld the search and Denson's conviction, they wrote that they had "little doubt that the radar device deployed here will soon generate many questions for this court".Intrusive tool
The Range-R sends out radio waves that can detect the slightest movements, including breathing, from as much as 50ft away.
Originally developed to assist US forces fighting in Afghanistan and Iraq, the radar motion detectors have proved popular with law enforcement agencies.
USA Today, which first reported the case, said that agencies including the FBI and the US Marshals Service, had been using radars since 2012. The Marshals Service had spent at least $180,00 (£118,000) on them, it said.
But none of the agencies has made any public disclosure about how or when the devices would be used.
In 2001, the Supreme Court ruled that police cannot use thermal cameras without a warrant, specifically noting that the rule would also apply to radar-based systems that were then being developed.
"The idea that government can send signals through the wall of your house to figure out what's inside is problematic," Christopher Soghoian, principal technologist for the American Civil Liberties Union told USA Today.
"Technologies that allow the police to look inside of a home are among the intrusive tools that police have."