Good advice in here already. One thing I'd add from experience: once you've stripped it back, don't be in a rush to close it up again. Get a cheap moisture meter and check the timbers are properly dry before reboarding — plastering over damp lathes just traps it and the mould will come back. A...
At that slow a rate of loss it's almost always a weep that's evaporating before it ever shows, so I'm not surprised you've found nothing visible. The quickest way to narrow it down is to isolate the boiler from the system overnight - if the boiler side holds pressure and the system side drops...