http://www.snopes.com/crime/warnings/carjack.asp

But that doesn't mean some idiot won't read it and think, "oh good idea"

Ever since the earliest warnings of this putative form of crime hit the Internet, we have been following news reports for any documented instances of an actual car theft (either perpetrated or merely attempted) that followed the script outlined in the widely-spread e-mailed caution, but we have yet to turn up evidence of so much as one. Were this "lure motorists from their vehicles by leaving flyers on their rear windows, then drive off with their cars" method as commonly in play as suggested in the example above, that surely would not have been the case. Nothing rules out there having been a few thefts carried out in the manner described that we have yet to hear about. But even if that proved to be the case, there is clearly no crime wave, no ever-present danger to motorists everywhere, no flyer-armed menace lurking in the nation's parking lots.

If I see money in the back window, I'll still drive home and look later.