I think we've all had moments like that, where you feel like some-one's walking over your grave so to speak. However there seems to be a logical and scientific explanation for most things.
Coincidence maybe, yeah a big one, or the mind can play some wicked games with us sometimes. Either way, the chances of it being a ghost are close to zero I feel.
As for UFOs, well that is a totally different kettle of fish. Are we really naive enough to believe that we're the only living race in the universe. As vast as it is, there is, I believe, something out there!