Minor quibble, but many of us have seen things in the sky that we can't ID. That's all UFO means.

My old (literally) landlady saw a UFO. It appeared to be three bright lights (or possibly one object sporting 3 lights) flying in close formation at sunset. She was startled and asked me "What is that!". It was Venus plus her totally messed up vision (she was in the midst of a chain of eye-surgeries). I don't have any idea how a point can split to 3 rather than 2 images, but she had a sharp wit (if not vision) and I completely trust her description of what she saw. This was a completely valid UFO, until I came along.

[Incidently... prior to this episode, I was completely skeptical that Venus could be mistaken for anything but a bright point of light in the sky -- not moving against the background of stars, at all. Live and learn!]