Happened a few times on our server as well. Don't have to interact with vehicles for it to happen.
With me it happened as I was looting my own body after being killed by AI. My weapon had disappeared (took me about 10 min to get back) but I could pick up my clothes as normal and to me it looked like everything was there. My co-players on the other hand saw me in underwear...
Naturally after restart everything was gone and I hadn't been in a vehicle since dying.