If you keep close to the wall in this small area, you'll hear a voice that speaks in utter gibberish. It's the belief of most players that the voice is that of Doug Rattmann who, by the time the events of Portal 2 take place, is long dead.
This last Rat Man den is, in my opinion, the creepiest. The den is unique in the way that it is the only one of the seven dens in which you hear the disembodied voice of someone who's been dead for years, decades, perhaps even centuries. The voice has sparked spirited debate, and several people have worked to decode the message the Ghost of Rattmann leaves behind for whoever finds this hidden alcove.
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The graffiti present in the den also foreshadows events to come later on in the game. At the end of the hallway, near the fan, there's a piece of graffiti that contains an odd poem:
The bell invites
Hear the turret for
It is knell
That summons to
Heaven or to Hell.
This poem refers to something known as the Oracle Turret.
Early in the game, while making your escape with Wheatley, you come across a turret that doesn't attack you when you come across its field of vision. In fact, it sits in a pile of junk. "I'm different," it says as you walk past. Doug Rattmann touches on this same turret in the poem found in the last Rat Man den. Later on in the game, while moving around the inner workings of the modern Aperture facility, you see the same turret about to be disposed of. Pick it up, and you'll be privy to a few snippets of information, some foreshadowing future events.
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As heard in the video, the turret mentions how Prometheus was cast down into the Earth's depths and pecked by birds for giving the gift of knowledge to mankind (Like GLaDOS was), and that "the answer is beneath us." Before Wheatley even mentions the fact that only the top layer of the facility has been explored so far, we're given vague hints to that fact from an oracle turret.
The information that the Rat Man den and the Oracle Turret provide, even when taken together, doesn't make sense until later in the game. It's one of my favourite moments because of the questions that it raises. How does the Oracle Turret know about the significance of lemonade here? How does it know all this? Could it be that this Oracle Turret is Cave Johnson, and that he was uploaded into a computer after all? It would certainly explain why they knew about the significance of Lemonade and the true identity of GLaDOS. And what about Rattmann? How did he know about the turret in the first place? So many questions.