Many people are going to say that this is just a 'Wheatley-Turret' as I like to call them, but think about it. *SPOILERS* During the final cutscene of Portal 2 while you are in the turret opera room, there are a series of rectangular-shaped lights on each ledge that the turrets stand on. But did you know that about 50 seconds into the opera you can make out a companion cube being carried by something? Or someone.

I believe that this is Rattmann going to the top of the lift to give you a present. Now what was the final thing that happened from Chell's view? So what do you think her present was? The charred Companion Cube that belonged to Rattmann. Rattmann climbed to the top of the shaft to bring Chell his Companion Cube. This couldn't have been GLaDOS because you can hear footsteps walking to give you the cube. And don't even try to say it was Atlas or P-Body. That just makes no sense.