OK, it’s as I suspected.
The map has sky behind a caulked face. This allows sunlight to leak through to edge luxels because sunlight is only shadowed by visible surfaces, not brushes.
If you put a textured face on the top/back of that wall between the staircase and the sky, the light leak will go away.
y
ah, that would apply to my terrain map as well, Ill test it and report back... I hope its as simple as this to fix 
Thanks Ydnar, that was the ticket for me as well! I guess we will have to improve our construction technique for this new version
RC
can the shaders used on our brush faces blocking light leaks, have surfaceparm nolightmap and nonsolid properties?
I assume they shouldn`t have trans?
I was still getting minor light leaks even after doing this (which I did before you suggested it since it seemed logical) - those are the leaks on the other thread about this issue. MOST of the leaks did indeed disappear though with externally textured faces interposed between world geometry and portal sky.