I had, sometimes, that some cars disappear. The disappear cars, always, they are a car that I enter on it previosly. If I close and open the game, the disappear car respawn (if you have the car key, It work).
I see, too, some duplicate car that respawn of "the nothing"...It respawn while I was driving the car. I stopped and revised the duplicate car, and, it have the some loot of original car. You can take the loot. After some minutes, the duplicate car disappear again.