Well, 4 Clipping graphs are loaded at a time (1 for each Z coordinate). I don't know if they all contain information, but if they do, then they are essentially loaded.
Try checking it out.
Clipping graphs can be gotten thru:
XClippingPlane[] = getMap().getRegion().getClippingPlanes();
Index it with a Z coordinate and grab the tile flags from the ClippingPlane. Tile flags contain information about each tile in the plane (104x104 tiles). Eg. if they're walkable.
If you think about it, it makes sense for all planes to be loaded. Some objects are rendered from different planes than they are actually located. Eg. if you're on the 2nd floor of a house, there are often certain objects that'll be rendered from the base floor, but obviously cannot be interacted with.