It's Alphonse's room in the secret entrance part (With the bed and fireplace and a safe). When I got into the room from above I noticed a safe there. But for some reason I thought the yellow werewolf change plate was to change back to MacGuffin (I didn't even just try the plate...), quickly used the map to travel to Mayor's Office to enter the secret entrance. But in Alphonse's room of course the other plate there didn't work... So I went back again but couldn't enter Alphonse's room anymore because the door to it in the previous room is sealed with red beams and the crates are gone (To put on a green button to remove the beams) because I solved that room.
I take it this is a bug? Please don't tell me it's intentional... Because now I can't get 100% anymore :(