I would definitely go to the MIL and tell her (and I'm a very non-confrontational person). She may not realize the droppings are there, and you'd just be stating a fact, not making any judgements. I'd appreciate someone telling me, if it were my house. And odd as it sounds, I have had the situation occur in my house, where droppings appeared overnight! It was in the spare room, and I went in one morning to find droppings all over the floor. I cleaned everything out, couldn't find any animals or entry-holes or anything! It was very strange. Haven't seen any animal poo in this house since then!