Okay, I think we just found their entrypoint. Our back mudroom had a lot of droppings...like 50 or so. There's also a bunch of stuff on the floor like sleds, snowboards, recycle bottles, Christmas tree, ornament boxes, etc. That back door leads to the backyard and is right next to our water heater enclosure. It is a bit drafty. You can see spaces and feel air coming through. When we bought the house in 2011, we thought of replacing the door, but didn't because remodeling costs were adding up. I guess we skimped on the wrong thing. For now, I'm going to stuff a towel between under the door between the mudroom and house. I'll decide what to do about the backdoor over the week. Sigh...