The room is very difficult to find without some sort of guide that includes several steps:
- Look for a large room with four tall stills in the center with a few more off to the side. Look along the walls for a ladder leading down underneath the stills.
- Climb down and circle around the stills to find an alcove with a mattress against the wall and a valve above it.
- Turn the valve to open the door into the secret room.
- Get out of the recessed area and climb back up to the main floor.
- As you get off the ladder, turn right — there is now an opening big enough to crawl through.
- Crawl inside to unlock the "Dev Graffiti" Achievement/Trophy.