Fall Through Platform Script?

I want to create a script that would allow the player to fall through certain platforms when pressing certain keys. My plan was, when the player was on an elevated platform and pressed the 's' and 'space' key, to turn off the player's box collider so he would fall through. Then when he exited the platform his box collider should be restored. The first bit worked but i can't figure out how to detect the player has exited the platform and enable his box collider, because the player can't detect he exited the platform if his box collider is turned off. Any help is greatly appreciated.