Some people (me included) had rapid drops in level in the first few months.
One cause could be air boiling out of the coolant. The coolant is stated to be 50/50 water. There will be dissolved air in the water component. When the coolant temp rises, and especially as it contacts hot spots where it will vaporize, that air comes out. Eventually it will end up burping into the overflow.
My previous car used a coolant concentrate that was diluted 50/50 with water.
To reduce the dissolved air (oxygen which causes corrosion and air in general which boils out at hotspots causing cavitation) I would first boil the (distilled) water first to drive out the air, then keep it covered while it cooled. I don't know how much this theoretical fussing helped, but, at 210,000km my water pump was inspected and was "as new".
