The reason you cant see behind some doors is because of area portals that have been setup by the mappers. they effectively cull out anything on the otherside of the door when its closed (geo, textures etc). The fundamental portal/leaf node system that is generated by quake allows for this culling to happen around specifically designed corridors as well. I suggest if you want to see it in action load a map in dev mode and use r_showtris and you should be able to see culling of parts of the level happening. (warning! not all maps have this set up properly which is why the r_speeds sometimes perform badly), this is why even with wallhack you cant see everything.

EDIT:Check out this thread they seem to point to a few resources that relate to this topic.

Build a wall of blockers can textures - but it has long been spared - there textural WH. That make the walls transparent. Just not all WH can not see through the door - there are many who quietly solve it. There are also solutions based on video card drivers - they just make the walls hardly visible. At one time we tried it with Panda to solve mapping in - but it was the wrong decision. The problem is that he WallHack built into the code of the game from the beginning.

I want to expose some thoughts:
1. WH is a built-in feature of the game's engine.
2. Any attempts to create an anti-WH based on shading system (such as using "areaportal" texture) are useless as they can be easily bypassed with writing other shaders and putting them into latter (in alpha-numeric order) *.pk3 archive.
3. To make a true anti-WH is to upgrade the engine itself. And servers should verify the engine's version (and maybe additional pak%.pk3).
4. Another variant is to make maps with transparent walls (of glass or ice).

Here's what you can do with the maps.
OVERKILL_TV

http://www.neytrino.com/#/4-content-00-02

Easy isn't inactive, he's just tired of people like you