We got to the end of Stonefort without encountering it so we turned around thinking it spawned in one of the stairs. We looked everywhere going back and got to the WP. Turned around again and made it all the way to the end for the second time. Nothing. We spent at least 10 minutes just trying to see if it would spawn by running near the walls and top of the stairs.
We can guarantee that the warden didn't spawn.