Just as I though when I read your post, you have a leak in your level. A hole that leads into the void outside.
I quickly found a leak (it may not be the only one, but OK). In the room with the ladder (or so it looks, it's a deadend room), you go up the small stairs. Then, on your right, look at the ceiling. You can see a small leak there.
When you fixed all of your leaks, your map will go trough the HLVIS and HLRAD process. HLVIS does some processing so HL knows what it has to render and what not, e.g. what can be seen at that moment and what not. At this moment, everything is rendered even whn you can't see it. HLRAD does the light processing. It won't look full-bright anymore then (IF you've put in lights off-course!).