This trick can be done at any time you are on the Atlantis. Go into your quarters. You will know that it is your quarters if you see some military uniforms inside the locker. Go to where your locker is located. Enable the ghost code and move around to position yourself so you can see the back of the locker. There is a license plate with the word "Sea Goat" on it. Make sure you are inside the ship before you disable the code by using the walk code.