Galvanic corrosion is slow and predictable: two different metals in salt water, and the less noble one wastes. Stray current is different. A small electrical leak, from a chafed wire in the bilge or from shore power, pushes current out through an underwater fitting into the water, and that fitting corrodes fast. People often call it electrolysis.
David Lynn, writing in Good Old Boat, names the shore power ground wire at a marina as a primary cause, along with faulty equipment on board or on nearby boats. Nigel Calder's point, also in Good Old Boat, decides the fix: stray current can't be stopped with zincs.
What a surveyor checks
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 and whether a galvanic isolator is fitted. Check the bilge for wiring sitting in water.
In the report
"Shaft anode renewed at last haul-out, now about 80 percent wasted; propeller shows pink, pitted bronze at the blade tips. Pattern suggests stray current. Have the boat's DC and AC systems tested for leakage before relaunch." See stray current corrosion.