If you watch the Ghost Hunters show TAPS, they seem to really try to be honest and I've never seen them looking like they are faking anything. Some of the footage and audio they have caught is very intriguing and makes you think. Some things that just do indeed look like what you would call a ghost, not a white sheet, but an image of a person. Some are black almost like shadows of a person. Some people believe that these are the demon dark entities.
There was one episode that could possibly give some evidence that these are indeed dead human spirits that are still here. They had set up their lap top computer in an area of this place, it had their various monitors of their cams etc., all cool looking in a way on the computer monitor. They caught a lady's voice that fairly clearly said "what is that contraption." Contraption is an old timey word for gadget or machine. To me this gives evidence that this is a dead person because that is the kind of words someone that died in the 1800s would use I think. Calling a computer a "contraption" because they would have no idea what it was. And I believe they had documented that there had been a lady that died there. (I think, some of the episodes kind of run together for me) but I do remember the voice using the word "contraption."
I think another possibility to consider is that these are people in a parallel universe that are existing in the same space as we, but in another universe that sometimes we can see glimpses into it. Which would explain why most of the sitings have ghosts with clothes on in them.

Just another theory.