One way that UFO's could be extraterrestrial
is if the "craft" that are seen aren't from another star system, but are interplanetary craft launched from a base or bases from somewhere within the solar system. I agree that it would be a waste of energy to send many expeditions to the same star system over and over again. Sending one and then setting up camp makes much more sense.
Lee, (Maybe Mimas really is the Death Star)