You can find a pair of cool posters referencing such games like Quake and original Doom from 1993.
At the beginning of mission 8 - Advanced Research Complex – turn left in the first hallway and enter the women’s restroom. If you look up at the ceiling, you will notice an open hatch to where you can double-jump.
You will appear in the secret passage that leads to a room covered in blood splatters. Look at the front wall and there are two posters hanging with logos of the aforementioned classic video games.