Ever since day one, my MY02 ML320 seems losing coolant about 1/3-1/2 inch below the coolant reservoir cool level line every 7k-10k miles. Dealer just did a pressure test and didn't find any leaks.

Is this normal for this engine? This is the first car I've own having this wired habit; a bit worried about the gasket now. Truck has only 20k miles.