How many times did you watch that amazing Portal 2 trailer? Enough to catch a glimpse of an old friend?
Take a look behind the busted supports on the right side of the screen.
I say "friend" because while you do form a bit of an attachment to the Companion Cube during the game, the unrelenting over-use of the little box as a meme eventually turned it into something of an enemy for many.
Let's hope that's exactly why it's there. It's now an Enemy Cube. And you have to hunt it down and tear its little pink hearts out.