Go down the area where you have to leave a clone behind to progress. The roof area directly to the right of the hole has a false ceiling in a part, I saw it because the area was slightly more grey.

I cannot for the life of me find any information online as to how you got there. Even a hint would be nice, I've been running around looking for fake walls for so long I found the message about game preservation that I don't hear anyone talking about. I agree with that message in a lot of ways, btw vikintor