I had a house that had a number of hanging plastic dome lights. When they cooled after being turned off, I would hear a pop and see a flash. A friend was convinced she would see ghosts. I decided to record the noise and the flash but it wasn't there. What was there was piezoelectric generator. I've found it interesting that many places with high ghost siting areas have piezoelectric geological effects and different cultural areas seem to have different results such as ghosts, UFOs or saints.