Thanks everyone for your comments and suggestions!

We did find the stench after all and I'm really embarrassed to say that it was a wet towel that was in the washing machine. Sigh. I thought the washer was empty since I had done laundry the day before. Oh well...

Nate - YES, Fritz knew. And he knew I knew he knew, and he knew you knew he knew. I kept asking him to use his superior doggie sniffer to find it, but he refused. I think he found the search entertaining.....or maybe he liked the smell. Hm.