Low pressure showing on boiler

[RoyWin]

What the easiest way to find why my boiler is losing pressure. It is about 5 years old Vaillant combi boiler, we have just had 2 vertical radiator removed and refitted for decorating and now the pressure keeps dropping approx once a week. I guess a leak under the floor is possible, or trapped air in the system as the system includes underfloor heating in the extension. I guess as well it could be a problem inside the boiler but all was ok before the decorating. Any help would be appreciated.