The following chapter of Call of Duty Cold War allows you to get to a restricted zone inside the base. This page of the guide will help you unlock new features in your campaign and an alternate ending.
During the campaign, in the base, you will find a door that can be unlocked. On the door on the right (as in the photo below) you will find a clue as to what you should be looking for in the base. It's all about documents and press articles which provide you with numbers needed to open the door.
You need 3 documents - a Clinical Record, a Warren Commission document and a Dallas News article. The first document will be hanging on the wall of the corridor to the left of the closed gate (as in the photo below).
The second document is located on the ground right in front of the door you want to open (it is illuminated as in the photo below).
The final document is about Dallas - you have to go to the darkroom to the right of the closed door. You will find the article on the right side of the room (as in the photo below).
In each document, the numbers you need to use to open the padlock will be highlighted in yellow. Given the order of the documents and the numbers included in them, you should be able to form a code - 11 22 63. This is the date of the assassination of John F. Kennedy, the US President. There's a radio inside, which may be useful later (depending on your choices during the campaign), a gaming machine and a computer that has all the game texts uploaded.