I watched [this video](http://www.physics-astronomy.com/2015/09/here-is-what-emc2-truly-means.html#.V4OxnRJywqK). At one point, the man explains how a flashlight loses some mass due to light emission when turned on. OK, I get that. Then he talks about a different situation, where the flashlight is inside a mirror box. I understand that this is a closed system, no light gets out, so there is no change in mass of the system. 

But there is one thing that I don't get. Suppose you put the mirror box with the flashlight on scales. In that case, after turning the flashlight on, the scales don't move. In the case of flashlight without the box, the scales move a little due to the loss of mass. So far so good. Now, suppose you have the mirror box on the scales and break one wall. It is no longer a closed system, light is emitted and the scales should move. BUT, how do the scales know what I consider to be a closed system? Do they move at a time the light leaves the box through the crack in the wall? What if the box were just larger, would the weight loss happen later? Or, if I had a box full of cracks and sent a beam of light through it, would the scales indicate weight increase at a time the light enters the box? This seems totally wrong, but I can't see a difference, from the point of view of the scales, between a cracked box with beam of light going through it and a sealed box with a beam of light moving inside. I just can't get my head around how do the scales know what to weigh...