Player not in hand asks to see losing hand at showdown?

[llleisure]

I know this has been asked but I can't seem to find the topic with search.

Situation was that I had AQo, raised PF, CBet whiffed flop, may or may not have fired again on turn. Checked river, player in hand turns over pocket 9s which win and as I'm about to muck player who wasn't in this hand at any point asks to see my hand. I should have just mucked because I'm 99% sure then the answer is no - instead I sort of dropped my cards and the dealer got a funny look, touched them to the muck and exposed them. Mmmmkay.

So is this common? And yes I know it's been asked a zillion times sorry, just trying to decide how I feel about it.