Search your house high and low for anything that might be rotting. They could have created a colony somewhere in your house. I had this happen before and I was ready to bug bomb my entire house until I found a pumpkin in the basement buried under some flattened cardboard boxes. It was the most disgusting thing ever! The smell was masked because it was next to the laundry room which smells like detergent, bleach, and fabric sheets. Good luck!