Did they do it as pax were boarding the plane, or did they just randomly walk up to people sitting in the gate area? If the former, yes, this is a favorite final act in the security theater performance. If the latter, no, I can't say I've seen that before... though they used to do random liquid checks. Would ask you to take the lid off your coffee (purchased in the secure area) while they waved something over it presumably to detect what was in it. I always told them to get bent.

I've also seen a variation of this rather often at JFK... after the ID check, they find 6 or 7 TSOs to line the jetbridge and do nothing but stand there looking menacing as pax file past and board the plane.