Not sure if anyone has noticed, but when you leave Atheon's room before you kill the Gatekeeper (yes you can leave it), and climb back on the wall most people use to skip the jumping puzzle , there is an alcove which you can go into. It is in the shape of a "T", and at one end there is a raised block with light shining on it. It is right after the "slip-spot" (the one spot on the wall where you get bumped off) on the wall if you're coming from Atheon's door, or right before it if you're coming like you just skipped the jumping puzzle. Sadly there was no chest when I went in. However, it might be a possible pedistool for a chest and I just didn't unlock it (perhaps the chest requires you do something first like the anti-templar-teleport chest). Also, not sure if it is there before you get to Atheon's door (aka before you clear the jumping puzzle part of the raid). Haven't been able to play since I'm grounded, but will post photo/vid if this is a place not already discovered and when i get ungrounded.