Without knowing for sure,..
I suspect that as Greggo says, you are now observing something that has always been happening.

I would just add that this is supposed to happen, and is nothing to worry about.
When the radiator pressure builds up, coolant will overflow into the overflow bottle from the radiator.
As the pressure recedes, coolant will flow from the overflow bottle back into radiator.
Well, that's how I understand it anyway.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...