What is most likely the reason for this? Air in the system that gets compressed when the engine is running, causing the level of coolant in the reservoir to be reduced. See pictures. I have not managed to burp the system. I seem to have a very small leak somewhere as I have to top up with abit of fluid two times a year. I have never had any problems with overheating etc.

 

Assuming this might be normal for your vehicle it may be a very small head-gasket or intake leak. Not to worry, some Alumiseal should fix this. if that is so. Get the engine nice and hot and add the Alumiseal to your cooling system and run it for half an hour around town. Alumiseal will NOT plug your radiator up at all.