It commonly confuses people when they change a radiator and fill it up that shortly after the radiator can take several more liters of coolant , its because your heater takes coolant from above the engine so it can't be filled alone , a hot engine expands and circulates coolant filling the cooler pipes , so yes trapped air is my guess though if topping up doesn't cure it I do recall someone having to replace the little valve thingy on the bulkhead in your engine bay that the highest about inch thick rubber pipes run through . Being so high i guess it can let air in without leaking coolant ?