Combi boiler lost pressure

[Jupiter01]

What would cause a combi boiler to lose all its pressure suddenly? I’ve not had this issue during the 4 years of this boiler. There are no signs of leaks around rads or anywhere else.

It’s now holding the pressure.

Thanks in advance.

 

[terryplumb]

Prv opening due to expansion vessel not accommodating system water expansion when heating in operation. Does pressure rise to around 3 bar when c/ heating is in operation ?

 

[Jupiter01]

Prv opening due to expansion vessel not accommodating system water expansion when heating in operation. Does pressure rise to around 3 bar when c/ heating is in operation ?

I’ve not been monitoring it but will keep an eye now. Also, this happened without central heating in operation as it has not been turned on for a few days.