Incredibly, after finding a hidden room in Jail Level 2, then again in Jail Level 3, Merrick found yet another one elsewhere on Jail Level 3. You can even see the other hidden room at the far upper left of the Mini-Map in this shot.

In this variation, the room is again 5x5, but this time it's five solid arches along each exposed wall. The middle panel again is the hidden door, which can also be identified by its particular bloodstain pattern.