To call it a mystery leak is that it leaks occasionally, but often when a gas engineer checks its source, he couldn’t find where is it from. A lot of cases, it is from AAV, auto air vent.
All combi boilers have a device called AAV. As shown above, an auto air vent (AAV) is to let air to get out, and to keep water in.
While air is getting out, the AAV is opening. When there isn’t any air, AAV is supposed to shut off to stop water coming out. But in a real world there might be some tiny particles in the system water. If there are some particles happened remaining at the seat of AAV valve, AAV is leaking, often at a slow rate.
As said to call it mystery, this leaking is often intermittent.
This leak is sometimes happened right after a good boiler service.