Setting off the metal detector gives probably cause for a search. I agree that for anyone who sails through the detectors, there isn't probably cause unless they fit a profile as determined by an experienced profiler.
I'd personally prefer the alternative of doing away with the detectors and allowing anyone with a CCP to carry, subject to ammo limitations - frangible rounds or AP, no hollow points, for example.
Setting off a metal detector isn’t probable cause. There are too many false positives. In fact, it is almost unheard of for the metal detectors to detect a genuine weapon.