If you see something flying which your mind cannot perceive or identify, I guess it's correct to refer to it as an Unidentified (to you at least) Flying Object. It does not however mean that it is some alien object or spacecraft from another planet. Back in the 60s a crew flying mainly IMC with just a few very short clear breaks filed an air miss with what was later determined to be the planet Venus! This is not the Air Canada story from 2011 but a different event.