A case for ghosts

Someone leaves a seat and a few minutes later you sit in the same seat.
You can feel the heat from the previous occupant of the seat.  Maybe it was a soft seat of a material like leather or plastic that holds the shape for a while so you can tell roughly the size of the person.

A person you know leaves a room and some time later you enter the room and recognize their cologne.

In each case although the person is gone, there is a residual indication left inadvertently by the person.

Birds have senses for direction that people do not seem to have, but there may be in some people a form of this sense which helps them to have a keener sense of direction.

So although we do not know definitely all the senses that human beings have, it is possible that some people have unrecognized abilities to sense things other people cannot.

It is possible that when people die, there is a residue left behind that some, but not most, people can sense. 
Possibly the residue is more apparent in some cases than others.

These factors could explain the perception of so-called ghosts.