Your logic is a bit flawed.
here is how it works - first have a look at the radiator cap - it has a rubber faced valve with a strong spring under it - you can push this valve with the palms of your hands and a hat shaped valve that you have to pull with your finger nail.
The heavy spring valve is the item that "sets" the system pressure at the rating stamped on the cap. As we all learnt in school that you can raise the boiling point of a liquid by increasing its pressure - thats why we have that valve in the cap.
As the engine runs - it generates heat and the coolant sucks it up - it then wants to expand (natural phenomena) but the cap is stopping it till the rated pressure (lets assume it at 14.7 psi above atmospheric) as soon as the cap sees anything above the spring starts to collapse and water leaks past the seal - this is collected in the plastic bottle on the fender. When the engine is shut off and the system cools down the liquid will contract and start to create a vacuum.
This causes the little hat shaped valve to be sucked open and water/coolant is sucked from the expansion tank. Both these phenomena happen every time you drive.
if you have a pin hole leak in the expansion tank pipe or the radiator cap neck or the cap on the expansion tank itself the system suck air from this leak (air is lighter than water) and once the vacuum is gone the activity stops. Now as you have air in the system the water/coolant will boil - and when you check it you will find low level under the cap.
Its quite simple