If you shoot all the small decorative UFO's in the shack, the screen flashes white and a gigantic UFO flies across the skybox. Not entirely sure if this does anything else as of yet.

From what I remember, the UFO's are: Two in the gift shop, one in the break room, one in the twin's room, one in the cellar and one in Stan's Room(this one might not be accurate, going off memory)