my guess is you have an air lock... air expands with heat.. so trapped air not in the header tank... but at some other point, expands and contracts as it gets hot and cold... that trapped air is there, because if fluid is leaking... its is replaced with something... either air in the header tank. or air getting in where the fluid leaks out.. or both..
its probably both because you top up the header tank, but still you see rise and fall of coolant level..
if you have a leak to atmosphere, and you bleed the system... initially you might see a slight improvement... since if you are lucky , you remove the trapped air..
but still until you have the leak.. the fluid getting out and the air getting in will continue.. this is because the system is semi sealed..
although the vent cap prevents air getting out.. until 1 bar... it doesnt like a vacuum.. so will let air in... and it does that as the trapped air contracts...
so will the leak point... if the outside pressure is greater than the pressure inside the system.. air will bleed in through the leak point or the vent cap
The second problem is the pressure in the system... as the coolant heats up.. it expands.. and as it cools down it contracts.. its not a pure hydraulic system.. devoid of expansion and contraction due to temperature or inside and outside pressure influence..
it reacts to them all.
so.. if you have a leak.. you wont get the correct running pressure dependent on how much fluid is leaking and when.. and what the pump is doing and what the temperature is..
its a very complicated thing.
and the sum of that.. is the fluid level rises and falls...
i know for certain now all that happens..., the gauge , tells me the influence of the pump on the system pressure, and what the idle pressure is with a warm engine and the pump running...
around 0..5 bar
thats 0.5 bar higher than ambient pressure at sea level.. and i live within about 150 feet of sea level.. at Holme Next The Sea
so if i get a leak.. at idle... its going to be pushed out at 0.5 bar , at whatever temperature it is..
and so on.. if i get a leak after hard drive on a hot summers day... and the coolant temp is high suddenly the leak lowers the pressure... and what leaks boils on exit and makes steam..
now.. maybe the leak is a pin*****... that doesnt change what happens except finding it is like looking for a needle in a haystack,,
the coolant evaporates as soon as it leaks into the atmosphere - if not it wanders along driven by air currents and engine shape et al.. until it evaporates or finds the lowest point and forms drops that drip.. somewhere else... it becomes gravity dependent as to where it goes.. and external engine temperature dependent on whether it evaporates or not..