I've been trying to find information on this ending, in the game's files there are images titled UFO Ending. The images are parody of Silent Hill 3's UFO ending with the art redrawn as the main character (in silhouette), Mikitaka, Caesar, and William, so I assume that accessing this ending might have something to do with playing as a Zeppeli descendant and interacting with Mikitaka in Cairo, but I'm not sure what to do beyond that (perhaps you need to kill a certain number of enemies and then talk to Mikitaka kind of like how you would trigger this ending in Silent Hill 3?). I'm also not sure if this ending is actually implemented or not. The Japanese wiki also doesn't seem to have any information on this ending, at least not that I could find by searching for Mikitaka's name or just UFO. Twitter also didn't provide any leads that I could tell, but not being a Japanese speaker, I could be wrong about it and the wiki not having information.

Sorry if someone has asked about this already and I missed it, but I'm just super curious about this if it is indeed in the game.