The exact same thing happened to me yesterday( he was actually shucking the bolt shut on purpose once finished), except I walked over to him and said something like: you're doing everything OK but you must leave the bolt open so it's obvious to everyone it's empty, I also explained about the fact he could buy himself a plastic flag but said as long as he's seen to leave the bolt open then all is fine. I said it quietly and used a lot of hand movement to make him see the point.I think we have far greater safety issues to sort out than release triggers!
A chap came out of a safety cage yesterday with a semi auto, he'd just shot a pair so the breech was closed.
I said " Excuse me your bolt is forward, can you open the breech please?"
He said " I know " and proceeded to walk past 15-20 people waiting , not one of them said a word.
There is no excuse for poor gun handling whether O/U or semi and that chap was clearly a bit of a tool but the demeanour of saying something has to be right to have the right effect.