The next thing to do is to locate the leak. If it is intermittent, then it is more difficult, but not impossible. Look for visual signs of a coolant leak around all the coolant hoses, water pump, radiator etc. Have the system pressure checked when it is cold and when it is hot.
This may be best left to an experienced Mercedes tech, they may know the right places to look based on their experience.
Here are some in your area: http://repairpal.com/directory?address=93551&car_brand_names=Mercedes-Benz
check on the left side of radiator, you may have a hair line crack , needs to be replaced, cannot repair, take a flash light to look and see whiel vehicle is running. go to www.discountautoradiator.com. I had to replace mine, cost on between $1235.00 to $150.00 for radiator