UK Car Repair Questions? Ask a Mechanic for Answers ASAP
I'd say that you're a bit low on coolant
as you do ' gain' coolant with it all warm as the fluid expands with temperature and so fills the system a bit more
if you have to keep topping up then
If you have a tiny pinhole leak in a radiator then these often don't leak water but will lose water as steam ( steam molecules being small enough to pass through the hole where liquid cannot) take a look on the rad for any signs of localised corrosion and for areas that look like they've been 'steam cleaned'.
If all of this is OK it may be worth replacing the header tank cap as again you can lose coolant as steam if this is blowing off too early ( the cap should hold pressure of around 1Bar)
The other thing to try is a head gasket / block test kit as these take a coolant sample and analyse it for any signs of combustion gas - if present this would indicate the head gasket is on its way out