I had one, and when I noticed it missing, I just assumed it had fallen off.
My first thought wasn't actually that someone had stolen it, but that I put it in a bad spot for wind, or that it had fallen off on a rainy day, or that I had stuck it on a dirty part of the car and it didn't stick so well. (Given that I don't wash my car often enough-this last scenario is most likely)
I took more care with the next one-washed the area where I wanted to put it more thoroughly-and I never lost it.
So, are there other circumstances that the magnets may not stick so well and fall off? For example, I always thought that if it got really cold, they'd get stiff and not adhere as well as they should.
Just a thought. I'm sure some are stolen- but many must just fall off, no?