In the very Southeast corner of the Unnamed city, against the South wall, opposite of where Shaleback Hollow is, there’s a hole you can fall into. The walls around it are all “cannot climb” type, and as such you cannot get out of this hole.
Not sure how to re-open ticket. We were not using mods when this happened, but now I’m using the Nilfheimr mini-map mod. I’ll try to get coordinates for you next time I’m on.