By an off chance there can be a vehicle rollback at some point. Happened to me and my mates. We had the VS3 and a military offroad stashed somewhere and somebody raided our camp and stole both the vehicles. We were pretty bummed but moved on and then later the same day after a server restart/crash one of our members logged in the vicinity of our old camp and the VS3 was standing in our old camp (the offroad was missing still). I bet the guys who got it from us were pretty unhappy about that since they didn't take anything out of it (or the items were duped). We then hauled it's contents onto a better server we found. So yes. Vehicles can disappear on you randomly. Your best bet would be to revisit where it was standing the moment you found it.