Orion's Arm's reasonably hard science fiction opened up my brain to how ridiculous a UFO would be. Humanity is "young" (9000 years since Apollo think?), and they have stuff like AI made megastructure sensors that can see movement on planets from lightyears away, hyper advanced and completely invisible nanotech that can govern planets, brains that span stars, and had it fairly early in their history. Even if our alien neighbors were that young (which is basically impossible), why would such a civilization have an atmospheric flying saucer? I makes no sense. Its like a caveman postulating that people will use smoke signals at 2023's tech level.