Turn your display gamma up really high so you can see clearly in the dark (works with nVidia Colors adjustment control panel to override darkness in any game), and then just as you enter the bunker, turn around and watch the outer blast door.

You will see two outer blast doors, one is stationary, and one is not. The moving blast door slowly "reopens" back into the area from which you arrived. Except since there are two copies, you will observe the mechanisms of the moving door pass through the stationary door....

https://imgur.com/a/AVty4dH