I did it during the final mission and am unsure where exactly it is on the map, but it's in the witness's monolith so go as east as you can on the map and you'll hit it
I couldn't get there without going into the mission due to the bosses guarding the doors and how it starts in a "locked" location. If you know a path, it may be worth a try. The mission (especially if you're over 1945) is a walk through in normal mode. It is just after the main ogre.
