How close did it get before it disappoeared?
The casual dismissal of UFO's may be wishful thinking, engaged in by people who cannot face this awful possibility. There have simply been too many detailed stories, for too long, from too many witnesses. The attempts to explain these sightings in some other manner are, generally, almost laughably weak.
Equally dangerous is the assumption that we should be seeking out aliens through the government-funded "Search for Extra-Terrestrial Intelligence" or SETI. As has been pointed out, contact between an advanced civilization and
a far more primitive one has usually or always, in human history, been devastating for the primitive culture. In the context of extraterrestrial contact, that primitive culture would be US, folks. Think about it. And that's even apart from the possibility that the advanced civilization might actually be hostile.