sabre combi boiler losing pressure?

Our combi boiler keeps losing pressure when heating is off, however if heating kept on constant it retains its pressure. When off it loses pressure within hours and closes down. I have searched for leaks under floor boards, radiators etc and no visible/audible leaks and no damp patches on ceilings, walls, etc where pipes run. It is a reasonably new boiler (2 years) and there doesn't seem to be any leaks in the boiler itself. Any ideas as to what could be the problem before I totally destroy my house checking every inch of piping.

LM
January 2010