UFO's are pretty common, as in UNIDENTIFIED flying objects. People see them all the time, but few take the time to identify them (my plane from Beijing some years back was delayed because of a UFO in the airspace).
What I'm getting at is that just because it's 'unidentified', doesn't immediately mean that it's aliens (which by its very nature would be IDENTIFIED, and therefore NOT a UFO!)
Markus