Bummer! Sorry to hear that. I know the feeling as it happened to me but a little differently. I had an expensive bike chained up. Thief stole the saddle only and not wanting to ride home without it I left it chained that evening which was a stupid mistake. (I had had a couple of pints which may have impaired my decision making ability.) They came back and took the whole thing later.
When I replaced it I started to do what jgnat's son does if I had any doubts about the area. But to snatch it right in front of you ... I think I'd have got caught out the same.