If you ask me, saying something is a "Ghost" is almost the same thing as calling something a "UFO." (Or at least, I feel it should be classified in rather the same manner.) A lot of people forget that a UFO stands for Unidentified Flying Saucer. But I come across that a lot (it seems quite often on the History Channel,) where a "UFO expert," upon exhausting all other possibilities, will call that "proof" that said UFO is an alien spaceship. Which, obviously, is a scientifically fallacious conclusion. It only means it continues to be unidentified.
Actually UFO is Unidentified Flying Object if I'm not mistaken, but yeah modern History Channel has almost become cheap entertainment it seems. I believe UFOs exist, but that doesn't make them alien life automatically, and alien life doesn't necessary have to be from outer space. Alien life is like unknown life.