A couple years back, there were some researchers that threw out a theory regarding ghost sightings in old haunted houses. They proposed that at least some older structures, particularly those that had been abandoned or unoccupied, were subject to the cultivation of molds and fungi that may cause visual or auditory hallucinations.
IMHO, it's a thought that bears at least some consideration and may also be at play in a dank Greek cave a few millennia ago.