Unvented system nightmare?

Hi, I hope somebody can help!

I have an unvented system that constantly drops pressure. There are no visible leaks (3 years dropping) and a couple of so called plumbers have tried to no avail to fix it. I have to keep the filling loop slightly open to keep the pressure around 2 bar. If not it drops to zero within a day. My hunch is that it is leaking into the water tank, is this likely/possible? As there is no other leaks anywhere.

Help!!