Assuming FR has the same kinds of ghost stories we do, the gaseous form, level drain, and ability to become a bat right before your eyes would all lead my PC, who has a decent INT score (note, old AD&D rule of thumb is INT X 10 = IQ) to discern that it wasn't just some nasty thief. But, the situation is obvious because anytime I roll my cursor over them it says "Vampire."
Duh!
[img]graemlins/thewave.gif[/img]
My PC might still joint them you know.