vokera combi boiler?

iv had 3 engineers out to my boiler because every four days the pressure had droped to zero. they checked the safety valve and replaced the expansion vessel, and it still droped every four days. they all agreed i had a leak however after taking all the floors up no leak was found. my plumber then capped off all the radiators except one which was not leaking and in four days it had droped to zero, we are at a loss what to do next.