It is probably a different level. All of the dungeon levels, house cellars, etc, are on the same map as the rest of Aleroth, Rivertown, etc, but normally the game hides areas other than the one you are in (even without the fog of war). Occasionally it does not, though. I once ran into a glitch like that on the map, took a screenshot, closed the map and reopened it, but couldn't reproduce the issue.