I have had a lot of this in my neighborhood lately
I would say 98% of the time, I lock my car and it stays locked. There have been a couple of times that I know for certain I have left my car unlocked.
One was when my wife needed to get something out, I unlocked from inside the house, she got her stuff and I forgot to relock.
That night, my car and 20 others in my neighborhood had been entered by someone and various things stolen. I got hit for loose change (maybe $5 worth). Others got hit for CDs, hats, sunglasses, and various other things.
The police caught the people that did it randomly the next day driving around in a stolen car and the trunk was full of random stuff they had hit.
About 3 weeks later I was washing the car, finished up the outside, was going to go back out and do the windows, kids sidetracked me, I got tired, never made it back out and forgot to re-lock the door. My car was entered again that night and again they took a few dollars in loose change (no, it was not visible) they also stole my leatherman tool (that pissed me off) and also took my nokia car charger and broke off the little door to the storage compartment in the dash (I was able to re-attach and it is fine)
Anyway, it is easy to sit there and say, lock your doors, but life is too busy to make sure the doors are locked 100% of the time. I have thought about putting an infrared camera in my front window pointing right at the cars.
The way I figure it, someone must be checking for open doors often in order to hit mine exactly on the two random nights I have left it unlocked.
The other odd thing, no one stole my escort radar detector in any of the goings on, each time it has been removed from the glove box and left on the seat or floor. I guess they are afraid of serial number tracking. I keep that in the trunk now, they have never messed with the trunk.