It is a puzzle but you got very far already. I don't want to spoiler but there is a "secret" room. If you compare the rooms you know with the map you might get an idea where it is. There are bricks in the wall that can be broken to get an access.
yeah, just found it. I had seen those markings before but expected the hidden exit to be in the same direction as its shown. so yeah, I was basically searching the wrong walls :-)
