To add serious insult to injury

There is a funky smell wafting from below our kitchen sink. We've had this problem before and we called the rodent people out in January to ensure there weren't any critters lying dead below. They told us there was nothing underneath. The smell has come back and is worse. I have cleaned out under the sink, bleached the cabinet, poured hot boiling water and bleach down the sink, and the funk remains. Our septic system monitor says that it isn't backing up.
We aren't really sure what to do now, but the house smells and my crankiness is hitting a new level.