There's an encounter at the Train Station from BGoTW that places a monster in the outskirts. What happens to the monster if the Terror Level is already 10? (I'm guessing it just doesn't appear)

Following the cold logic of AH, Id say put it on the Train Station with the hapless investigator and engineer...

I'd say put a monster in the Outskirts. The rules don't say anything about there no longer being an Outskirts limit: if enough monsters get put there by odd effects (Mother Hydra, for example), then you could possibly overflow the Outskirts, empty it and raise the terror level by 1, which means add a doom token to the doom track.

If this seems like it's not much punishment, then consider the fact that you're already at TL 10 so you're probably screwed anyway.