A door code is required in the warehouse you go through when finding the second book the royal bunker doctor needs. You will not be able to get the code until the Tower of London mission has been completed. You now have the ability to read the Enochian language. Return to the warehouse and clear the area around the keypad. Next, enter the office beside the keypad room. Look through the glass towards the keypad and notice that the Raven of Dee above the keypad door is incomplete. Also there are some incomplete pain marks on the glass. Move around so that they line up, then scan it to get the code. Note: There are four digits spread out in the bunker. Besides the one in the mirror by the door, you can find others by following the trails revealed by your blacklight.