I've been having this problem too. I've lost somewhere on the order of $60K worth of vehicles.
Also, keep an eye on your car if you leave it parked on the street. I had a Bravado Gauntlet STOLEN as I was taking a picture of it. Dude just hopped in and drove off! If you let it get out of sight, it's gone forever!
XBL Gamertag: JWRebholz
PSN ID: jwrebholz
...No, I'm not very creative when it comes to this sort of thing.