The mod has been in development for 13 years.
In the chapter on the track near a crashed tank is a secret developer room that contains health, armor, ammo, and a computer monitor showing a screenshot of the player standing in the room.
In the chapter weak force you can find a tv monitor in a recording studio with a picture of Rick Astley's foot from the never gonna give you up music video on it. If you interact with it a cover of never gonna give up plays.
In the chapter hostile takeover you can find six dormrooms with the names of valve employees on it. If you press all the buttons in 20 seconds the door for Gabe Newell's dorm will open were you can find a radio from the portal games.
The name of the chapter hostile takeover is a reference to the canceled Half-life hostile takeover expansion for Half-life 1.