From what I heard
cases of "ghosts" usually happen when the person suffers a violent death. Otherwise there seems to be no cases of ghosts at all when someone dies.
Perhaps the violent death causes damage and allows a copy of the neural net to leak out? The witnesses who claim to see ghosts, actually end up with part of the neural net trying to bond with their brain and it causes them to see things temporally. But it does not work out and eventually the neural net breaks down and they stop seeing things. It is not a ghost they see, but an hallucination. Which explains why others cannot see it, and some can.
