boiler pressure drops to 0 when boiler off?

have vokera unica he boiler almost 4years old recently noticed pressure dropping so simply topped it up now pressure dropping to between 0 and 0.5 daily advice please my plumber simply says he will have to take all the floorboards up to find a leak he does not appear to have checked anything on the boiler itself. There is a copper pipe leading to outside wall this is dripping water outside when boiler operating Any advice please