Weird internet advisory... but it actually makes some sense
My mother just sent me a chain email... man, I hate those things. But this one actually made sense, so I'll relay the basic premise:
A new carjacking tactic is to affix a piece of paper to the rear window of a parked car. It should be small enough to not be noticeable until one checks the rear-view mirror prior to backing up, but large enough to be noticeable when one checks.
In theory, the driver then gets out to remove the paper, and the carjackers spring, grabbing the unlocked car, key probably in ignition, engine probably running, and for women, with purse probably in car.
I don't know if this is based on any actual occurrence, and I tend to shrug such things off... but this one seems like it would work.