After serious thinking RJH wrote :
My gas combi's sealed system pressure drops from 1.0 to about 0.5 bar every
few weeks. It seems to stick at 0.5, but I've not left it long enough to be
sure. A few seconds on the loop valve and it's back to pressure - so at most
a
pint or two of water.

Check the PRV (Pressure Release Valve), usually the exit pipe is
outside - put a plastic bag tight over the end, with an elastic band.
If that collect some water, that's where it is being lost.

Does the pressure increase, when the boiler is burning? Could be that
the expansion vessel has no air in it, so as the pressure rises, it is
forced out the PRV.