Remote gun detectors

A few weeks ago, the New York Times reported that the NY Police Department was working with the Department of Defense on a remote firearms detector. According to the article, the detectors are  presently effective at a 3 to 5 meter range at finding guns that are being carried concealed. The objective is to improve the detectors so that they work from a distance of 25 meters.

Commentators, what do you think of this? Does is raise Fourth Amendment concerns? Second Amendment issues? Any other constitutional or policy questions?