People are weird.  Anytime I have anything worth more than a couple of bucks and the car is parked, it's either out of sight (short term) or I'm taking it with me (car is parked for more than an hour or two).  I'm just paranoid and automobiles just aren't inherently secure, not to mention that people have a tendency to forget to lock their cars when they get distracted (or choose not to do so because they're under the impression that once the vehicle is on their property that it is inherently "safe" from idle hands).

That said, unless those plates are stolen, I don't think the police will have too hard of a time tracking down the thieves.