Hidden Areas in editor

[FlawlessHappiness]

My editor seems to have hidden areas that i can't see.
Though, this only happens in the main maps.




I cannot select them, nor see them.
I just want to see how they did it. Why can't i?

[The chaser]

Weird thing. Maybe they just made planes without applying a material (sometimes happened to me).

You say that you cannot select them?

[FlawlessHappiness]

No. But i know there is something up there, since i saw the easteregg in a video.. And i can see the billboards... but cannot select them

[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.

[FlawlessHappiness]

YES AWESOME!!! Thank you! You are a genius!

[Cranky Old Man]

This is a common editor feature in general level design: You hide the areas that you're not currently working on. Working on a huge level without this aid, would be somewhat of a mess. It was a while since I worked in the editor, but I think they divided the levels into horizontally sliced layers, resembling floors.

[FlawlessHappiness]

It doesn't look horizontal to me.
When i press + it just cuts off at the Z line