Law enforcement is getting new surveillance tools. But they don’t always want to talk about them.

At least 50 US law enforcement agencies are using hand-held radar to "see" the inside of houses. The radar guns are just the latest in a long line of tech tools being quietly deployed across the country with little public scrutiny, raising questions about how the Fourth Amendment applies in the digital era. The radar uses radio waves to detect even slight movements inside a house.

The version used by the US Marshalls Service -- L-3 Communication's Range-R -- is a handheld device with a range of up to 50 feet, according to the company's promotional materials. Law enforcement agencies started purchasing the devices more than two years ago, according to federal contracts uncovered. But their use was largely kept quiet until a December federal appeals court opinion revealed that officers had used one before entering a house to arrest someone wanted for parole violations. Officers had an arrest warrant, but not a warrant to search the home, alarming even the judges.