At the end of the map, where the bots get killed, if you shoot at the 2 dots up there, so they change color ...
... the glass will break at that thing on the right
Then go stand on it/shoot it and the door will open
It's hard getting in the room, because after the door there is that slide thing that launches the bots away. I only managed to do it with the scout. After you get on the other side, theres a long walk. After some time, you'll fall in an invisible hole in a room full on sentrys
If you manage to avoid/destroy them, you'll then have another short walk in a hallway where you'll get teleported back to the main map and this guy will appear
And sorry, the images turned out rather dark, I don't know why