madpogue 

There was a NYT feature article a few years ago, about a bike shop proprieter somewhere in NYC. He mentioned he's had occasion to do that once in a while when someone comes in attempting to sell / trade-in a bike. He was a pretty big guy, and he would just put one hand on the top-tube and invite the person to stay or go, as he calls the police.

As for the general subject matter of this thread, the "stranger's" story would have to be extremely convincing. There are too many ways to game such a system. Some PO could sell the bike, follow it as it changes hands once or twice, then show up as the "victim" of its "theft" several months/years prior. Or maybe some former roommate, co-worker, etc., who happens to be as knowledgeable about your bike as you are, might get a wild hair to mess with your life, co-ordinate with some other party you don't know, feed her/him enough intel to convincingly show up as the "victim". IOW, even with a convincing story, it's waaaay too complicated to make the call based solely on what the person presents. I don't really have an answer, other than that, because it can get so complicated and because of the imaginable (and unimaginable) wrinkles, there is no blanket "what would you do" answer.