If the pressure on your boiler is going up too high for no obvious reason maybe the filling is passing when its closed.
When not in use the valves on an external filling loop should be closed, the metal braided filling loop removed and the valves capped. The majority of people don’t do this so there is always a chance that the valves can be opened accidentally or be faulty. This can allow too much pressure to develop in the system which may cause the metal pipe near the boiler to overflow.
Closed valves but pressure still going up
If the valves are closed and the pressure is still going up it can be the filling loop valves failing to close properly. Listen closely to the valve if you can hear water flowing with the valves turned off the valves have failed and will need to be changed. If you can’t hear water flowing and are confident in your abilities remove the filling loop and see if any water is flowing. Even a slight drip from the side attached to the mains can be enough to raise the pressure over a long period of time. When you take the metal hose off there will be water in it, this is normal. You are looking for a flow of water or a drip from a valve.
If they are passing or even dripping the valve should be changed. The valve on the central heating side of the filling loop tends to be the most commonly changed. This is probably the difference between a valve sitting in clean drinking water and a valve exposed to dirty chemical filled central heating system water.
Replacing the filling loop involves turning off your mains water supply and draining some of your central heating system. A replacement filling loop looks like this.