We  had the same.  Comes and goes.  Eventually we discovered it was dependent on another boat near us--  it would happen when the other boat docked andhooked up to shore power, then disappear when it left.  Months later, in another location it happened again.  I have had the feeling we were ending up detecting a leak in a nearby boat (boats at most marinas share the same ground, even use your zincs if they don't have them).  I have no idea how this is possible, maybe some of you with more electrical knowlege do!  We also have a galvanic isolator with detection as well and no correlation with its indicators.  The only time our galvanic isolator showed a leak was in a questionable marina in Greece.  Tests with a volt meter showed the galvanic isolator was solving the problem even though indicating a fault.  The Amel indicator showed no fault. BTW the addition of a galvanic isolator slowed our rudder zinc loss to near zero.