Cardinale says scanning a person for weapons isn't a search at all, likening the technology to traffic light cameras and speed guns, neither of which requires reasonable suspicion in order to be used.

The question of whether a certain technique is a search or frisk under the law largely turns on the subject's privacy expectation. Courts generally require police to obtain a warrant before entering a home because, even in tiny New York City apartments, people expect a modicum of privacy there. In 2001, the Supreme Court ruled that police use of thermal imaging devices to check homes for the telltale heat produced by high-intensity lamps used in indoor marijuana-growing operations constitutes a search under the Fourth Amendment and is "presumptively unreasonable" without a warrant.

Even on the street, where a person has a lower privacy expectation, the law requires some minimal basis—reasonable suspicion—for stopping a person and searching him. That includes scanning for weapons, according to Price. "The Fourth Amendment doesn't vanish when you leave your house."

Kelly says that, no matter how extensively the devices are used, they won't replace typical, old-fashioned police work. As he described the scanning project in a speech this month, he added: "We aren't relying on technology alone to achieve our vision of the future."