Pressure loss on Vokera Boiler?

I am losing pressure on my Vokera HE Compact boiler. It was installed 18 months ago. The first winter it ran fine with no problems. This winter I am losing pressure regularly, topping up every 10 hours or so. I have had floorboards up in every room and can find no leak. I have fitted valves on the flow and return to each radiator and turned them off individually, but still can't find the problem. Where can the water be going as it must be a pint or two per day.
When I run the domestic hot water the pressure drops but returns when I switch off. Also when I run the cold water it dips momentarily and the returns. Is this normal ?
I recently went away and left four radiators switched on, with my son keeping an eye on things.After a week the pressure had not dropped. When I came home and tried other radiators the pressure dropped again. When I returned the system to the same state as it had been on all week it then decided to lose pressure again. Frustrating !!
Is there anyway the domestic water can affect the pressure as the hot water was not used that week. I am at my wits end wondering what to try next