You have air in your engines cooling system/radiator
If you have air in your cooling system it will expand as you drive the vehicle and the engine gets hot.
When the air expands it eventually finds it's way to the highest point in the cooling system which is the radiator cap. The radiator cap has a spring loaded seal within it. When the pressure in your radiator and cooling system overcomes the spring tension the pressure will get released into the overflow tank. The released pressure may take the form of some liquid coolant as well as expanded gasses that are in the system.
Additionally, the radiator cap will allow fluid to be drawn into the cooling as the engine cools down and that gasses and liquid create a negative pressure in the system. Essentially the radiator cap acts as a two way valve that helps to purge the system of gasses.
The end result is, if you have air in your cooling system it will eventually purge the air out and replace it fluid and draw this fluid from the overflow tank.
The diagnosis is that you have air in your cooling system that is getting replaced with the fluid. This is normal operating procedure.
Here is a video explaining how this works.