Boiler losing pressure every few days for no clear reason

What we told them

Pressure loss with no visible water means the water is leaving somewhere you have not looked, or it is not leaving at all. Check the pressure relief discharge pipe outside — if it is damp, the expansion vessel has lost its charge and the system is venting every time it heats up. That is a twenty-minute repressurisation job, not a leak hunt. Only if the discharge pipe is bone dry should you start lifting floorboards.