Start up the coop hub - which you can do with the developer commentary if you can't find anyone online - turn left and jump towards the door for Course 4, falling to your death as you just miss the light bridge. But look left as you fall. Even though you die and hit the bottom of the bottomless pit, you can clearly see a signpost in the fog.

Anybody else seen this? Anybody have a video of it?

Good catch. I didn't get a video of it but I did get a picture of it; the fog makes it hard to see but if I get the chance I'll decompile the map and look at it in Hammer.

I looked at it, both in-game and in the decompiled coop hub in Hammer, and it seems the model name is "silhouette_cube_side." The texture is pitch-black. No idea what it might actually be, but clearly it's been there a while as it's in the decompiled version of the map I have from several months ago.

I don't think it seems significant, but you never know.

I have a feeling the model is used in the "super 8" interactive thing. Certainly doesn't look like it'd be used in Portal 2.

That model is used in the wheatly chambers as well, hanging of the testchambers underneath you.