It was only a matter of time before technology to detect the presence of a gun on a person at a distance became a reality, but they are working on something in NYC to do just that. You can certainly bet Bloomberg will want to step up this project, in the hopes of landing more innocent gun owners in prison.
There is already arguably Supreme Court precedent in this area in the Kyllo v. US, in which the Court ruled that using special technology not available to the general public requires a warrant. Kyllo, however, involved the home, so the Courts would have to extend that protection to the streets. But it’s not inconceivable that would be the case. Bloomberg has never been remarkably concerned about American liberties, however, so it doesn’t surprise me this is coming out of New York.