ferroli modena 102 pressure loss?

I have a ferroli modena 102 and recently it has started to lose system pressure. it started to do it occasionally but now when you top it up you can see the pressure drop as it goes down. it doesn't go to zero though only to about a quarter bar.

when the system is on for either hot water or heating, the pressure stays up.

there are no obvious leaks at radiators and it has been working fine up till now, is there anything on the boiler to check?