Zincs can be affected by a multitude of things that have nothing to do with YOUR boat. An old engine block close to your slip, a steel boat or two on the docks, a leaking shore power connection on one of the dock pedestals. The list is endless.
When I started sailing, there were still boats with copper sheathing on their hulls and if a steel boat came into that marina (or vice versa), everybody's zincs would go ballistic.
It was my understanding that the alloy zincs were for fresh water only. Perhaps that thinking has changed? If you really want to know what is going on with your boat, a surveyor or marine electrician will have a meter for measuring your 'leakage', if any. Call one and get your boat tested.