Secrets
While venturing the game, several secrets can be found in some locations and hidden areas that are left from the developers.
The Dev Room (Alpha/Demo)
The Dev Room is a not-so-secret area in the original demo level from Carrion's Alpha sneak peek.
It can be accessed after acquiring the dash upgrade, if you make your way back to either the turret room or the room above where you will need to destroy some wood on the left to go down. In the turret room you will see a vent leading into a little jungle-looking area where if you keep heading to the left you will run into a room blocked by a bit of wood. Destroy the wood and head into the room, there you'll see a door that you can interact with using <space>, that will take you to the Dev Room or "Phobia Game Studio Headquarters" where you can have fun slaughtering the humans there and which you can leave by returning to the door you came through in the first place.
Wolfenstein 3D on Computer Monitors (Alpha/Demo)
If you have a careful eye you might spot that on some monitors of the computers the humans are using, they're playing quite a classic game, which is Wolfenstein 3D to be exact. You can spot it from the game's blue walls and classic font.