Combi boiler drops pressure

Hi I have a Worcester 35CDi Combi Boiler that drops around 1/2 a bar pressure each week for the past 2 months. I keep topping it up expecting to see evidence of a leak but nothing so far. I have checked the pressure relief valve is not leaking. I am a bit lost to know what to do next. I don't want to start pulling all my floors up until I can confirm it is a leak. It would be helpful if I could isolate it to an area.
Does anybody have any advice on how I could trace the issue/leak.
Many Thanks

Have you checked the expansion vessel?

I f your pressure relief valve is definitely not passing (not a trace) and your pressure does not go over say 2.5 bar then your pressure vess will be ok,if so remove the combustion front with the boiler OFF and check the auto air vent for leaks, any spay etc will be obvious if thats ok have a good look up from underneath behind the bulk head valves, the 35 has a lazar welded back panel this can leak from the back and if its a pin hole leak just soak into the wall so it should be stained by now, if thats ok its up with the floor boards

I didn't suspect the expansion vessel for the reason that continuous filling meant the water had to be going somewhere. Would this be an incoorect assumption.
I have had a good check around and behind the boiler but can't see or feel any signs of damp. If this now means lifting the floor I wanted to best guess where to start. I have assumed if there was a leak upstairs I would see stained ceilings or walls by now so it may be on the ground floor under the house. Does anyone have any hints like listening to the pipes to find the pipe run the leak is on.
Thanks for the advice so far.

The size leak you are looking for is VERY small. You may find it almost impossible to see.

As it is so small. what about adding some internal leak sealer, (similar to radweld). You can inject it into a radiator bleed valve.