maybe it wasn't a deed done by a human but some sort of small animal or pest with enough muscle capacity to fling it over? were the windows/doors/openings closed or open? of course it isn't a possibility if they kept the house extremely clean and also if they didn't suggest the same right after it happened because if it's happened once it must've occurred in the past as well. anyone will feel somewhat uneasy after ruling out such possibilities even though there probably is a reasonable explanation why