The screen wash is a horizontal sensor, and the magnet floats in the water, up against the sensor - When the water runs low, the magnet drops away from the sensor, telling CC it's low. And again, these can get gunked up, and that gunk stops the magnet getting close to the sensor.
The only thing I can think of with yours flashing up occasionally when booting it, is the water is moving around in the bottle, and theres gunk on the magnet, which is stopping it getting fully in contact with the sensor, but enough to keep the CC happy, and when the water moves about in the bottle, it's pushing the magnet away just a little bit more and CC thinks it's low. Maybe just a clean up with solve it ?
Worth a shot, it's free