So everything is bled and fine with the coolant
Had tank/top hose/ radiator/thermostat changed a while back
However after around 3-500 miles the coolant light comes on if it is not topped up. Can't see any leaks from the usual places
When it does come on and you drive it home- wait for it to cool and open the cap. There is pressure from the tank and to refill you have to re bleed the system
The headgasket is fine- its done this for as far as i can remember
Im guessing there is a leak which is drawing in air when the coolant is low and the car cools?