Cops responded to call that two people were displaying guns in store.
Asked dude to step outside to talk since officers were observing strange behavior.
Cop followed man outside and saw gun in back waistband. Cop pulled gun out.
Subsequently found out he was a felon in possession of a stolen gun.
From the facts you stated here, I don’t have any problems with this search. “Observing strange behavior” could be almost anything, but at some point we have to give our police officers latitude in cases which may be a life or death situation. They were already called on the scene with two guys displaying guns in a store, that doesn’t sound like concealed carry, that sounds like they had them out with possible evil intent.