Coolant needing top up often on the aveo 1.6 G3

[Brit01]

Hi all,

I have a 2011 aveo G3 1.6 and it does seem to need its water reservoir tank topping up every couple of weeks after heavy use (lots of start stop traffic).

I have checked all fittings/pipes and there are no leaks. Oil is good and not changing level or looking milky. No smoke. No smells inside.
Everything is running smooth. No leaks from the water pump. I live in a dry dusty area so it would be quite visible if it was leaking.

Is this normal? Just evaporation from the release valve on the reservoir cap or could the cap need replacing? Yellow cap says 17 psi on the top but knowing the reliability of this GM car I´m not too convinced.
A closed system should not need topping up so much I believe.

Would be great to hear from other people´s experiences with the aveo coolant level.

Thanks