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.
Bob and Suzanne, KAIMI SM 429 In the water in Malta