Galvanic corrosion is slow and predictable: two metals in salt water, the less noble one wastes. Stray current is different. A small electrical leak, from a chafed wire in the bilge or from the marina's shore power, pushes current out through an underwater fitting into the water. It can eat a fitting in weeks.
Where it comes from
David Lynn, writing in Good Old Boat, names the shore power ground wire at a marina as one of the primary causes, along with faulty connections or defective equipment aboard or on nearby boats. He also notes that boats with isolated, unbonded fittings can still suffer from external stray currents. His own sign was zincs, shiny and new, almost entirely eaten away.
Nigel Calder's point in Good Old Boat is the one that matters for the fix: stray current cannot be stopped with zincs.
How a surveyor checks it
Compare the anodes with when they were fitted: fast wastage is the first clue. Look for damage concentrated on one fitting, a shaft, a propeller or a through-hull, rather than spread evenly. Ask whether the boat lives on shore power, whether there's a galvanic isolator, and whether anything changed when the problem started.
With the boat afloat, an electrician can test with a reference electrode and a meter, with and without shore power, and check the DC system for leaks with everything switched off.
What decides the grade
Damage. A fitting, shaft or propeller badly eaten is an A, because it can fail. Fast anode loss or local damage that's not yet serious is a B. A past problem with sound fittings is a C.