No unusual smoke coming from the tailpipe, indicationg that the coolant is entering a cylinder (A.K.A. head gasket failure)?
Check around each hose connection carefully for any small leak; coolent returns to the overflow tank, as long as there are no leaks, as coolant will take the path of least resistance. Coolant will leak there, as well as air will be drawn from this leak, rather than the overflow tank.
I had a Ranger pickup that had a low radiator, and was adding to it once a week; the overflow tank was at the proper level, yet the radiator never drew from it. I finally located the leak; one of the automatic transmission lines that connect to the radiator was loose, and I could see the tiny mist of coolant spewing from it when I let the engine idle for several minutes.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.