Combi boiler is loosing pressure every 2days.not obvious leak?help?

Bazza has the right idea. You are unlikely to find the sort of leak you are looking for with the system running hot.When its cold do as Bazza suggests and take pressure up. I have found that some of the newer style rads weep around the vent plug and cap on top of the radiator. They are fitted with rubber 'O' rings which fail.