Derp... that sucks. It was there when I posted it. I don't have the screenshot saved anywhere else and I'm well past that area without a nearby save. It's in the first courtyard of Caer Lem where the six brimstone vipers attack. You go past the two crates you have to push out of the way, up the stairs and right before you turn right and there's a trap there... the ledge is up and behind you, on the side of the tower sticking out of the wall, covered in a bunch of greenery.
The same tower sticking out of the cliff sideways that the characters comment on after arriving in Caer Lem.