I'm assuming that an unused building would not be well lit. One class of things that might be mistaken for ghosts in such a building is various animals. A fami

I'm assuming that an unused building would not be well lit. One class of things that might be mistaken for ghosts in such a building is various animals. A family of raccoons moving farther into the darkness to hide when a human opens the door, for example. Or a lot of rats or mice. Another would be humans who are clandestinely using the space. A trail of moving water running down a wall from a water leak might reflect light in such a way as to appear to be some sort of shimmering ethereal thing moving around.