So I’m not sure, but there may be something required to do before you can unlock this. I had already finished the game completely (all alarms, extinguishes, etc) before I found this, so there may be some requirement. I think there is, just because of the fact that I didn’t notice this while looking for collectibles.
How to Get It
All credit goes to ZFire2020 !
Really simple. Go into level 4, Blackout. Go all the way to the generator like normal, but do NOT go through the elevator to the end of the level. Instead, start backtracking the path that you came in from.
When you get near the door that is the entrance to that final room, a chess piece will pop out. When you get closer, he will disappear. Keep going all the way back to where you crosses the pit on the wooden bridge.
Once there, the chess piece will drop down from the ceiling like batman, then fall into the hole. This hole used to just teleport you back to the other side, but now there is a new room down there. A lower pitched version of Dr. Glenn Pierce’s garbled speech from the end of Clone will play.
After going through that door, you will have 2 paths and a radio. Going straight will put you back at the entrance to the room. The path to the right will be closed, but if you interact with the radio, the Standard Orientation Protocol will open it for you.
Once you walk through, you will end up in what looks like a control room. There is a chess piece sitting at a computer. Once you enter, he (or she) turns around. You are then sent to the game normal game credits, except there are pictures of cute dogs as the credits go by.
Was this on the PC version?
Yes I just played for the first time and decided to go back and you don’t need to hit all the alarms or fire extinguishers