Draw Calls

Even though objects are behind other objects (and thus, not visible), they are still drawn, you just don't see them because objects are then drawn in front of them. In order to cut back on the amount of draw calls, look into Occlusion Culling. (unity pro only)

If you don't have unity pro, look into creating a class that hides game objects on a trigger hit, and another that unhides them when it's hit, then have an array of public gameobjects for each trigger, and have the array reference all the gameobjects you want hidden/unhidden. It's a hassle, but it's an okay workaround if you don't have unity pro/can't use occlusion culling.