Combi boiler heat exchanger leaking
If the filling loop has been removed and capped so you are sure the combi boiler is not filling from the mains the heat exchanger is the only remaining cause. Combi boilers have a heat exchanger that takes the heat from the boiler and transfers it to the cold water coming from the water mains. Modern heat exchangers are made from very thin stainless steel in the form of flat plates.If the steel fails either through corrosion or manufacturing faults the mains water can mix with central heating water. The hole that is caused by the corrosion/fault is normally very small so the pressure rise is generally over several hours or even days.
The heat exchangers can’t be repaired so have to be replaced. This isn’t a DIY job so should only be done by a registered boiler engineer.
fixmyplumbing 
I had this fault and two engineers could work it out and just fitted extra expansion tanks and other stuff that did not fix it etc. Eventually I phone Grant and a very helpful guy said exactly what you have written. It makes me mad that I then had to tell a registered boiler engineer how to fix a fault, but we do seem to have a boiler engineer mafia in our area.