After testing this on a couple of other sims, it turns out that it's some kind of script conflict which is making the vehicle disappear. I've been able to reproduce this 5/5 times on an empty sim, so I think it's safe to disregard the abuse/griefing element of the previous question. However, for my own knowledge, and for the knowledge of any future seekers, I'd still like to know if it's possible for a 3rd party viewer to delete or take other people's vehicles(or any other object for that matter) out from under them. Anyone have any ideas?