Darker pieces of wall generally indicate a secret passage. Often you'll end up on the other side later. I suspect it was the intent to open these secret passages from one side (the one harder to reach), but several of them don't seem to work. I've found one such passage in the serpent temple, but via the kitchens and the elevators you can get on the other side (you'll have to get off one elevator half way up).
Even in the asylum there's a wall that won't open, and there are several chests behind I found no other way of reaching them them doing some intricate jumping in the room with the 'open' stairs. (It's the room with water in the middle, a stairs leading up to a higher level in the south, and a higher level in the north which I found no way to reach except with jumping while looking up.) Looking well at these darker parts of the wall revealed two more secret rooms in the crypt, after the kerah statue. These you could open with small buttons hidden by crates.
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