Well, you haven’t found it yet, but there is definitely a leak somewhere. Things to check: dishwasher and washing machine. Wander around the house and listen for the sound of running water. Go around the outside of the house and look for wet places or lush growth — if there IS any growth at this time of year where you live.
If there is a way into the main drain from the house, open it and check for running water when nothing is being used.
You might even ask around for a plumber who has a leak detector. Hope that helps.