I recently decompiled a few official maps to study and I came across some things I can't explain. There are these big brushes than span an entire area, practically sealing them, textured in toolsblack or a fake skybox white. I've seen them in King and Barnblitz. At first I thought they were the buddy brushes to an areaportalwindow because there are also areaportal textured brushes in the same spot as them, but they don't appear to be tied to a func_areaportalwindow. Maybe I'm just selecting them wrong but I use the select tool to select them and alt-enter to go into object properties. No matter if I'm in group, object, or solid select mode the object properties only show that it's a solid with 6 faces and that it's in the areaportal visgroup. Here's what it looks like in Hammer, King and Barnblitz respectively.