Kevin Boyd for the R Street Institute: In a growing trend begun a few years ago without much public disclosure or debate, at least 50 police departments across the country — including the FBI and U.S. Marshals Service — now deploy radar devices that could allow them effectively to “see through walls.”

The technology consists of hand-held devices that send out radio waves which can detect the slightest movement, even breathing, from distances up to 50 feet away. Other advancements currently in development could be mounted on drones and other vehicles and could possibly map out a building’s interior and locate people inside it.