How about option D, don't do anything this time but warn Chris and your other neighbours to keep a look out, say you'll do the same for them, and that if he is about he's not meant to be there, is likely to be theiving and they should call the police.

The problem I see with the piece of mind option is you're basically just haranguing a man in the street who'll likely flatly deny it and where does that get you? And the problem with the police option is that it's a bit excessive for the crime and likely isn't going to achieve anything either.

Whereas if he does it again, then police definitely.