2003 BMW 325 water leaking

I had to add coolant to my car two weeks ago for the first since having the car for three years. This past sunday I had to add water again. There seems to be a slow but moderate leak. When I check the water reservouir the stick indicator below normal range. No warning lights come on display. Please advise before taking the car to the mechanic as they seem to add more problems if you don't seem to know the the initial problem is. Otherwise the car is running fine. I had service on Dec.08