Another car I had in my base flipped after server restart, had plenty of room, no items on floor nearby. I don't get it.. I took a screenshot, I'll try to upload.

I have to add: The server just restarted, I had a truck in my base, full of gear, when I logged back in, it was completely gone, and my other car sitting on its side?

sorry, retract my last statement. I found the last backpack without guns in it..

I just reproduced it again, had a pristine backpack in my hands, not 100% full, no guns in it this time. Dropped to fight off a horde of zombies (I believe this is another bug - they all alert if one zombie in the area alerts, and then you have 10 zombies attacking you at once), backpack disappeared. It was a tactical backpack (63 slot) so can see it pretty easy when it is on the ground.