I think it’s important to remember that UFO stands for unidentified flying object. This is any object that can’t be identified. It could be an alien saucer or just something that no one can identify. Many times there is perfectly plausible explanations.
An article was published by the NY Times which is generally a very trustworthy news source about a group of Navy pilots who saw something that they couldn’t identify or explain. They said it was moving extremely fast. They even managed to get video of it. The object was at 30,000 feet and moving at sonic speeds. These two factors make it very tough to come up with a plausible explanation. It’s probably not fiction.