Yes there are a million fantastical scenarios one could imagine. But they are all incredibly implausible compared all of the mundane, boring non-alien alternatives:
* People getting bored and imagining things (“seeing faces in the clouds” so to speak)
* People being primed to see something (they read about other “UFO” sightings) and then, of course, they “see it” too
* People wanting to believe something for tangential ideological or social reasons — the government is corrupt and is hiding things -> UFOs must exist!
* People literally just making things up (they want attention, money, want to one-up someone, want to be special, have unregulated emotions, etc)
* Faulty / mis-calibrated sensors
* Data corruption / misinterpretation
* It was just a bird / cloud / water vapor / reflection of sunlight / electrical short / optical illusion
* And so on
These are the types of “boring” things that are always the real explanation when someone starts talking about crazy shit like ghosts or aliens. Aliens and ghosts are cool and exciting so people don’t want to believe the boring reasons. We often want distractions in our lives, and what a fun distraction that would be, eh?
Wild and outlandish claims require overwhelming evidence.