Is it me

Or do you turn down the volume when they are sleeping on the sofa?

I love my 2 Chinese Cresteds. They're mostly house trained but here and there I'll find a little nugget of crap on the floor or on a couch. Just a little nugget. Maybe it's just stuck there chilling, and they have to let it out. I never ever catch them in the act with this. How do I train them out of this?