Through thousands of years of evolution we have developed instincts to protect us from danger. These instincts can be triggered by otherwise safe situations when they match our built in list of warning signs. The burst of chemicals being pumped round our system readying us for the fight or flight response heighten our senses and awareness of the situation around us, this can provide us with what you could call false positives. Once you get into this frame of mind you begin to be more concious of stimuli you would normally ignore as background information. Also, sensory deprivation, such as complete silence or darkness doesn't go down well with our minds. We are constantly receiving information and when this stops our mind starts inventing things to fill the gaps, seeing things, hearing noises, etc.
So no, I do not believe ghosts exist, but I do believe the (more subtle) reports of sightings are real but misinterpretations of reality.