Also might want to check the electrical connection to the level sensor. Might be that it's not making good contact and therefore misreading that as a low level.
A friend's '95 525iT did this on her coolant level sensor. Had plenty of coolant, but the sensor has become slightly disconnected. Re-connected it so it was then making good contact and the message went away. Might be having the same thing happening on your washer fluid level sensor.
Cosmos Black/Black 1995 540iA M-Sport
BMW Individual Exclusive Edition, 1 of 65 total, 9/25/95 Build