Hi all - was losing coolant - not much - top up required every few months - carried out visual - nothing evident - added some fluorescent dye and used a blacklight - still no evidence - replaced the cap on the coolant reservoir ($20) - seems like its fixed the issue. Its a basic enough action but just something not to overlook before diving in with coolant system component replacement etc..... Car is 9 years old now so understandable.

 

The coolant contains H2O so a poor seal on the cap might allow water vapor to escape without leaving signs of a leak, other than gradual loss of coolant volume over time.

 

Thx Merlin - yep that could be the answer - for years coolant level never moved - then a couple of years ago got the warning light on the instrument panel - topped up then monitored coolant level more closely & noticed top ups were required more frequently - so hoping the new cap remediates the situation. (Its reasonably likely due to the lack of leaks evident in the system - ref the fluorescent dye)