More than likely, you dumped your tanks, but then there as still a bit of water left, or else you added a bit and then drove to your next destination. By leaving 3 or 4 gallons in the tank and driving, everything sloshed and pulverized everything when driving, thus water blasting the sensors and removing whatever was caught on the sensors.
If you dump your tanks before traveling every time and then add 3 to 5 gallons of water with a bit of laundry soap or Dawn Dishwashing liquid, the sloshing of the water and the soap will keep your sensors clean. But if you don't dump and travel with "stuff" in your black tank, you run the risk of crud getting caught, getting pulverized off, and then getting caught again. So if you drain the tank first before traveling, there's nothing to catch on the sensors except pulverizing water and soap, cleaning off anything that could potentially get caught on them.
