the previous owner may not have filled it with antifreeze, there is a little plastic thermostat on the coil that has a tendancy to leak, when it does it will leak antifreeze into the washer bottle, make sure the thermostat isn't leaking, mark the level with a marker and drive the car or pressurive the cooling systen, if the thermostat is leaking the water level will rise.
if it is it can be replaced seperately, it is mercedes number 126-830-07-84, when it leaks most people bypass it, but i think it should be repaired correctly