My sister has this ghost thing too. She shows me pictures all the time with these little "mysterious orbs" in them. Most are taken at night with a flash. I'm trying to tell her that it's the flash bouncing off of the dust, insects, airborne whatevers... she insist on the ghost! I told her to take a shot of the rain at night with a flash and it'll be a "rapture"!
It's kinda funny you should mention that, I was talking to a photographer the other day. He was spending a beautiful day fixing flashlit snow flakes in PS.