A sky won't block sight to faces. So whenever you can look at faces that are in the pvs (possible view set), you see these 'trough' your sky.
In fact, I would consider it as a designers 'fault'. Since when you look out of a window and you see the interior of the next hallway, you should normally see the outside of the building... so, as the designer, you should create a wall there instead of just having 'sky' and think that'll do it...
Trigger_camera's are used for cinematics, like in Half-Life when you were carried away by grunts and thrown in the crusher. That was done with a trigger_camera.
In CS, trigger_camera's are used for the several viewpoints you see before the game starts.