Baxi combi 105 HE keeps loosing pressure?

There are no leaks on my heating system and as far as i can see there is nothing coming out of the pressure relief pipe. It only takes the smallest amount of water back into the system and the pressure needle shoots back up to 2bar and the boiler works perfectly. There is no pressure loss when using the hot water. The system looses pressure every day after it has run?? can anybody help with the diagnostics please thank you in advance from a very grateful retired builder.