As you already know, it's gotta go somewhere. If possible try pressurizing the cooling system (15-20 psi) with shop air. This sometimes uncovers leaks that you won't find when the car is standing still.
Take a look at the spark plugs. Look for one that's cleaner than the rest. This would clue you that you're losing coolant through the combustion chamber.
95 E320 Cabriolet, 131K