You must find the secret room in the Distillery to earn the Dev Graffiti achievement. When you reach the room that contains several tall stills, search for a ladder leading down to a recessed area of the floor. Climb down to this low area and follow the path around the base of the stills. At the end of the path is a mattress; interact with the valve just above the mattress - this opens the door to the developer graffiti room. Return to the ladder and climb up to the main floor. At the top of the ladder, turn right to locate a small opening in the wall. Crouch and crawl through the low entrance to the developer graffiti room.
There are a few Easter egg rooms (all awesome) but the one for this achievement is in the abandoned distillery. It involves turning a handle by a bed that's covered in ammo and pick ups that then opens a grate which can't be missed.
It's in the Abandoned Distillery on the 2nd disc. There's a big room with some numbered stills (look like big silos) and its in the back left corner...You'll see a small set of steps and a opening you can only get into by crouching.