If the warning light is on before you start the car, and goes off after you start the car, and then comes back on after some driving time, the sensor is most likely OK, performing as is should, and responding to reservoir level.

If so, the remaining question is why is the level dropping. Probably air in the system, but how much, and where is it coming from? Ah, that'll be the mystery.

Come, Watson, the game's afoot!

Best Regards,
Jim