I had a fully loaded police truck when I saved and exited the game. It was parked and turned off about 20ft from my location. Loaded up a few hours later and the car was gone. This was parked by the rear exit to the movie theater by the mall.
EDIT: Another odd effect. The backdoor to the movie theater is now locked and it was NOT when I last played. As far as I can see, the pile of dead zombies I was disposing out back are still there.
So, car and contents vanished, door locked itself.
The trunk was nearly full with a few tote bags, a school bag, and a hiking bag loaded mostly non perishable food, building supplies, tools and some guns and ammo.