There are 3 things that you need to check.
1. Tank and pump (for leak location)
2. Hose (connecting to pump)
3. Sensor/float
(1) You'd want to look at the tank carefully for leakages as it usually leaves dirt and water marks around the area where the pump connects to the tank. If you're seeing this on the outside of the tank, you're likely to be replacing the pump. This is easy to identify as you'll find yourself topping up washer fluid every other day.
(2) Run your fingers at the connection point between the hose to the pump. If it's wet, check for cracks at that point. You can cut the end and just reattach the new end to the pump. Securing it with a cable tie might be a good idea as well. This is not really recommended as the solution doesn't last very long.
(3) If water maintains at proper level, then it's the sensor/float. If you have dismantled the whole tank/pump/sensor before, you'd realise that the float on the e36 is capable of cracks, and water tends to flow into the float itself (being hollow). Yes, it's possible just to replace the float, leaving the rest of the part intact.
Good luck.