The spot on the keel looks like it is above the joint between the sump and the keel, so there would somehow need to be a path through the hull itself for moisture to get there. Look inside the bilge and see what might be going on. Perhaps there's a crack in a stringer or floor? The moisture aft might somehow be coming through the stern tube or stuffing box. The way it spreads up from there, however, may indicate a problem more similar to the one forward. Rain dripping down the topsides can also create damp patches in the antifouling, but that usually leaves traces showing the path it took.
