Portal 2 Easter Egg - Referance to Ending

On Portal 2 Chapter 2 : The Cold Boot, in testchamber 7 after you pick up the companion cube three times look to your right, near the corner. For a split second you can see Wheatley in a hole, but he'll quickly cover the hole up with a panel. But that's not all. After you see it pick up the companion cube and turn up your volume. (WARNING THIS CONTAINS SPOILERS) There should be music. And if you have played Portal 2 all the way through you know that GLaDOS sets you free with a show; Hundreds, maybe even thousands singing one song. The companion cube is actually faintly playing that song. It's pretty cool.