Hidden Areas in editor

[The chaser]

Today, by accident, I think that I got the answer.

Go at the bottom of the level editor, there should be something called "Clip planes". Now, open the dropdown, and there will be a plane called "1". It isn't enabled, though. If you enable it, a part of the map disappears.
Which leads me to think that Frictional Games did this for performance.

I don't really know much about what are these, but hope it helped.