Took a bit of time, but I managed to get into the window I was talking about. The first 2 pictures shows the path you need to take. Run and crouch jump off of the pipe that leads to the window, you're aiming to fit into the window. You may be able to mantle on, but not from the pipe below it and if you're moving quickly it'll be really inconsistent. Took me 15-20 minutes of trying to get inside. Once you get in, keep going. You'll eventually end up where you need to be. Once you've committed, theres no way back. Make yourself comfortable, theres little else you can do.
Found out about this secret courtesy of The Voices Network on youtube.
Run along this pipe and crouch jump^^^^
You're in. Go deeper.
Commit. Deeper.
Welcome Home.
