I knew about Santa Claus. Just got thrown for a loop because this is my seventh save file and the first time I ever noticed a UFO.
In my headcanon, the Juminos aren't forest spirits, they're actually an alien science team. They're using you to gather a variety of biological and geological samples for their survey of our world. That could be one of their spacecraft.
I've seen this several times on my current save. Was also wondering if it was significant of something.
Dwarves aren't exactly confirmed to be aliens as far as I know. It's just one persons hypothesis which is seen as a pretty silly suggestion to most according to that lost book about it.