Many of the apparitions (or ghosts) that people have reportedly seen nowadays at reputedly haunted locations hardly seem to resemble the classic stereotypical image of a full-bodied, animated spectral figure of a deceased person. Instead, they seem to take rather ambiguous forms, appearing as shadowy outlines, floating lights ("orbs"), or misty, indistinct blobs. It remains a possibility that these supposed "ghosts" may have a purely physical explanation, and do not really represent the spirits of the dead.
However, there have been a fair number of haunting cases reported throughout the history of psychical research in which witnesses have encountered an apparition of a deceased person that seems to be in line with the classical image of a ghost. And in some of these cases, the apparition was said to have been seen in the same surroundings, displaying the same action(s) time and time again, as though it represented a lingering "trace" or remnant of the deceased person's presence. The term residual haunting has often been used within the paranormal community to refer to this kind of experience.
How might we come to understand the phenomenon of residual haunting? This question has been pondered by psychical researchers ever since the early days, and one possibility they considered is that this type of haunting experience might represent a kind of location-specific "place memory." What is the basis for this concept, and how might it help us understand the experience of residual haunting? An article which begins to address these questions can be found at this link. (NOTE: the article is in Adobe PDF format)