Bear with me because I'm a Unity n00b and my knowledge of 3D engines is more than ten years old. :)
Back in the Quake days there was talk of 3D engines using "portals" to separate two volumes in the game space. Basically, the engine would render everything in the same volume as the player and ignore everything outside that volume. It was an optimization trick and usually was used to separate the interior of a building from an "outdoors" terrain. When the player was in the terrain area the interior of the building wouldn't be rendered. When they entered the building the outdoors area wouldn't be rendered.
Does Unity do something like this? I've read about occlusion culling and it sounds like it may be an up to date way of achieving the same thing.
Thanks for the help!