Well, conservation of mass states that they cannot simply disappear. Matter cannot be destroyed. I say they're just flash-liquidated and absorbed into the surrounding walls, where microfactories within the walls manufacture them into recycled blocks. It's a very, very complex and pointless system, created by a savant of a scientist who greatly understood how to achieve the pinnacle of recycled processing, but had no idea how to apply it to the everyday world and better the lives of those around him with his genius.