Falling Water-Level in the Cooling Syst.

For quite some time now the water level in the cooling system of my 300E drops gradually. I've been topping it with about 0.6 litres of water every 2-3 days. I've taken the car to a mechanic who, after thorough investigation, could not discover any leakage point - I've even poured some colorful coolant to see if that could help detect the leak point (if any) - but no success.
* What could be the possible causes of this problem and what solution shall I apply?
* Could water be evaporating from the cap - the temperature guage never gets above 90 degrees

Thanks in advance for your suggestions.

Oge.

300e 1986