Corrected entry: At the very beginning of the game in 72 Hour Mode, when you make it to the security room after the incident in the Entrance Plaza, Brad tells Otis to seal the door tight, making it impossible to enter or exit the door for the rest of the game. However, in Overtime Mode, which takes place right after 72 Hour Mode, the door is now open.
Captain Defenestrator
23rd Jan 2008
Correction: The Army forces the door open when they arrive in 72-hour mode. You can use this door after that point in the 72-hour mode as well.
Captain Defenestrator