Pressure drop on Baxi Combi 30HE?

Above bolier losing pressure to the tune of 2 bar every 8 hours. Expansion vessel changed, secondary heat exchanger changed. Boiler located in integral garage with all pipework running through ceiling void, pipes run down for downstairs radiators and up for upstairs radiators. Leak sealer has been added. No visible of signs of any leaks in house and this has been going on for 3 months. Can anyone give me any idea where I go from here - pressure testing of boiler and heating system on an individual basis using isolation has been inconclusive. HELP!!