I wonder if a larger animal has found a secluded place to do its business, and like you said, your MIL went outside and moved the cover back.
Several years ago, a local high school was vandalized during a summer vacation to the tune of $100,000 in damage :eek:, and someone had defecated on a teacher’s desk. They used DNA to figure out who it was (and yes, it was one of the vandals) and the funny thing about this is that the culprit tried to blame his twin brother (who had also been arrested). That didn’t work, because they were not identical twins. I worked with a few people who went to that school, and after that story came out, they were talking about which teachers’ desks they would have liked to have taken a crap on.