There are many ways how to explain that:
1) The first thing the came into my mind when I saw the screenshot from the newsletter was, that it could be catacombs under Mystrock. The skulls doesn't really fit into this, but still...
2) He could simply escape.
3) Maybe, when the change to non-human happened, he was outside of Mystrock(although from what I have understood from MM so far, people from Mystrock rarely leave the city, especially nobles).