So, as it turns out, to get this achievement is a lovely bit of fun.
Step 1: Get S Ranks in neighbourhorde mode. I believe you only need to get an S Rank with every character in the mode but I got on S Rank on all maps too just to be sure of what I was doing. Doing so will give Gordon a Tape Cutter on start of a new game.
Step 2: Start up a new game. start a new game and go through the intro sequence as normal, all of us should be aware of where this is going, but just to be clear, there is a taped shut closet in the sound stage changing rooms to use the tape cutter on, doing so will reward you with 'The Odd Key'
Step 3: Profit. Go back to the hotel, and head upstairs to the end of the hallway, use the key on the door on the left and head inside. The achievement will pop, and you will now have access to a developer room with all the goodies in the game.
Happy Hunting Achievement Hunters.