Actually there is a far spookier version than ghost being spirits left behind. There is a rather cool theory that can explain such sighting as well as repeat-sightings that appear to interact with the viewer.
It states that what is happening is that the viewer of the apparition is him/herself stepping into the same space or viewpoint of the 'ghost' and that what you are seeing and feeling, ie cold, frightened, happy, nervous, is what the 'ghost', of the space you are presently occupying, felt and saw when they too stood there. That's why there are so many toilet and stairway ghost as there are many more opportunities for the viewer to be occupying the same space as a 'ghost' and therefore stumble into the same atmospheric conditions enabling you to 're-experience' the 'ghosts' sights sounds and feelings. When someone thinks they have been strangled by a ghost at night and then find out that someone was murdered in that same room it is not the strangler that has returned but you occupying the same space as the victim which enabled the victim to communicate its feelings to you if you should be receptive enough to pick up on them. If you see a ghost train or car then you may be occupying the same space as one of last passengers to see it before its crash, etc. It would explain why many people see ghosts who smile at them, or chase them, or think they are someone else. Perhaps you are 'inside' the ghost yourself and are merely seeing their reactions based on who they see when they see you. It also explains why there are more ghosts in older homes as there are many more experience-sites that you could stand or sit in - hotels too.
Eh?
I often wonder if we all moved our furniture back to surrounding the fireplace instead of the telly (wherever we've chosen to put that) if we'd see more ghosts. Mind you. It certainly puts you off going to the loo at night. Or sitting in 'grandpa's chair' etc!