Hmm... I'm not sure I buy the 'invisible human' ghosts but there is a theory I find a little more intruiging;
It's been suggested that materials absorb energy and of course, since the human body is controlled by electrical impulses it uses electrical energy to fire off emotive responses... so why, if the energy used to prompt or create that emotion is strong enough (read: a large amount of energy, not a ghost who had been a overbearing father in the 1800s) could it not transfer to the walls of a house or a ceramic vase?
Surely that a little more possible than 'invisible human beings running around playing with people'? I mean it accounts for the not being able to see anything when someone comes across a 'vibe' or 'feeling' about an object or place, doesn't it?
Also horror films don't help much, they tend to convince people that if some place is dark it probably holds a demon/spirit/ghost/etc and combined with a human fear of the unknown it creates all sorts of odd thoughts...
Final note: Most Haunted? Pah! "Oh oh... it's getting colder over this side of the room...!"
Duh, you're standing by a 130 year old window in the middle of a cold night, might it perhaps be draughty? Oh, there's a hot spot in the room? Check for water pipes and radiators... :roll: