These rooms wouldn’t have doors. The only way to notice them would be to spot inconsistencies in the interior layout. You might think at one point “This room is oddly small, considering it shares a wall with the previous room. It doesn’t add up.” Or perhaps you’d get a hint that there’s a hidden room if you’d hear noises coming from there. You would need a tool to break the wall if you’d want access.
Alternatively, add (un-disarmable) trap plates in the mansion, and stepping on them would open a nearby secret room by removing a single wall tile. (keeping in mind to add exceptions in case of various destruction scenarios, where the trap plate or the door gets destroyed). Low Perception characters might trigger these by accident.
Alternatively. cover the room entrance with a line of bookshelves, and you’d have to drag away a bookshelf to reveal the entrance. In case monsters are placed inside the secret rooms, monsters might break through the bookshelves, potentially catching a player off-guard. Also add more bookshelves in mansions, too.
It’s an open question what these rooms should contain. Rare books? Stashes of old weapons? Jewelry? Art, even? (Although the game doesn’t have paintings, does it?) Skeletal remains and spider webs? Old maps, marking the location of a treasure or a secret temple maybe? Messages that give hints that there are further underground levels in the mansion, and the downstairs into them can be revealed if one knows which piece of furniture to displace?
I sense potential for something lovecraftian here…
Of course, hidden rooms could be a part of other buildings as well. Homes, cathedrals… any others?