First: thanks for clearing that up, it has been too long since I really delved into the blueprints of the house. I indeed kinda was under the impression that one room was left, then did a quick tour of the house, and when it wasn't upstairs (I thought it was sissy's room) despite technically needing to know it was the toilet my brain fell into "no that must be what I remember" excuse mode.
But that leaves my theory what could be in there in really dire straits. Or, mathematically speaking, if something isn't true, then any assumption about it is correct, and my theory is right