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You can find the graffiti room at the Subway Town when you're on the job at the abandoned distillery.
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 still. At the end of the path is a mattress, interact with the valve just above the mattress, this opens the door to the developer room. Go back up the stairs and look to your right, you see the door open half way, crouch and enter the dev room to get your trophy/achievement.
This video will show you the Secret Developer Graffiti Room location that unlocks an achievement or trophy in the game once you enter it.