Warning, some spoilers below!
The Swan Station has two entrances, a hatch and a door.
The hatch was sealed shut, and it took dynamite to access it. It then opened up to a 50 feet vertical shaft with a ladder.
The door, on the other hand, could be easily opened from the inside. Later on, it was unlocked and could be easily opened from the outside.
So what, exactly, it the point of the hatch? It would be very inconvenient, so it probably wouldn't function well as an alternate exit, even before it was sealed shut. It also wouldn't have a point security-wise, given that the door was fairly noticeable and presumably easier to access.