Holding tank sensors
Sounds like an electrical problem to me. The lights on the monitor panel are activated by what I call a "contolled short". Simply put, the sensors are wired together and there is a ground and 3 positive probes. When water or substance gets on the first probe, it shorts to ground and the first light comes on. As the water level rises, the same thing happens with each probe and it is shown on the monitor panel lights.
If you are getting the same reading on every tank, full or empty, I would suspect the monitor panel has gone bad or has a bad ground. It would be a good idea to check the grounds at each tank, but I really doubt that could be it.