Cooling System DEAERATION
WARNING: This page is about a different car, the 2016 Dodge Durango. However, it is still accessible from the selected car via links, so may be relevant.
As the engine operates, any air trapped in cooling system gathers under the radiator cap. The next time the engine is operated, thermal expansion of coolant will push any trapped air past radiator cap into the coolant reserve/overflow tank. Here it escapes to the atmosphere into the tank. When the engine cools down the coolant, it will be drawn from the reserve/overflow tank into the radiator to replace any removed air.