No, the usual culprit of a rough break is stomatid snails, they move fast and get in through small openings and tend to go unnoticed. These are the small black half shell snails that look a lot like a slug. Besides that I really don't know what could cause it. I assume you are using the screen and that is why you are sure nothing entered? I am puzzled as well as the plastic is not exactly fragile, it takes a fair impact usually to cause this.