I have been looking around the other forums and one guy said he no-clipped (flew through the walls) to look for it and he thinks it is the
incineration tube that Glados drops you down right after you wake her up, and I agree.
I just played that part again and just after
you find her still dead you walk behind her, jump down those broken stairs, walk over some cat walks and are now under Glados. If you look around you cant see the shaft that leads to the incinerator in witch you will soon be falling. I think they may have made that into an infinite fall.
