Are you sure whether the radiator cap has been replaced? The cap is the device that allows coolant to be sucked into the radiator from the overflow reservoir as the engine cools. If that's not it, then have the dealer put a light vacuum on the cooling system. It's possible(although remotely) that the cooling system could have a leak that allows air in, but blocks coolant from escaping. Have you noticed if the volume in your overflow reservoir actually increases?
Try to follow the
KISS method as you figure this out (Keep It Simple Stupid)!
