Where is the water in my cental heating going?

[WaterWalter]

Had renovations in my house last year and since then there's a few weeks where it's fine then the pressure drops and has to be topped up at the boiler and then the system drains completely and has to be re-filled. Earlier in the year the plumber found a leak and fixed it but it's happened again. Plumber thinks there must be a leak somewhere but can't do anything to find it. There is no obvious water that we can see but it is a big system with UFH in the kitchen.
Is a mystery leak the only possibility or could something else be causing a complete loss of water in the system? Is there a way to find where the leak is coming from if there's no actual water leaking anywhere visible?

You can turn off the boiler and let it cool, then turn off the isolation valves on the boiler.
Then if the pressure drops still it must be the boiler. Otherwise it may be elsewhere on the system.
If the ufh has isolation valves or flow pots you can close them and try the same again.

Thanks for the tips. How would I know if the UFH has isolation valves or flow pots? There's nothing visible and the plumber has never mentioned them. Are they fitted as standard? (It went in last year.)

Would be fitted as standard, there should be a manifold with the ufh pipes coming out the bottom, and usually a pump on the side. The top usually has a row of flow regulators which can be screwed in to remove the flow, and the return has a electric valve to turn the heating on and off with the thermostat.

There isn't anything like that. The way I understood it is that the UFH is just hooked up to the same pressurised system as the radiators. There's definitely no pump and no manifold that I'm aware of, nothing like that under kitchen sink or in garage.

Fair enough, there has to be a method of mixing the water temperature down so the floor isn't damaged by the direct boiler water, but maybe there's another way that your system handles that