for those who are adding coolant regularly but don't see any leaks or white smoke on the exhaust, one possibility is that you need a new radiator cap. if the spring on the cap is already weak or if the rubber seal is leaking then the coolant will overflow into the reservoir. but you will not see any leaks since this will most probably happen while you are riding in the car. then when you have stopped the engine the coolant level in the reservoir will go down as it cools, that is why when you check it again you will find the coolant level in the reservoir normal, but the level in the radiator will be low.

mura lang radiator cap. it is best to replace radiator caps every few years anyway.