On a similar note, last spring after putting the boat back in the water, I had a similar. I tore apart the plumbing system expecting to find a faulty thru-hull (but all was intact). After 3 return trips I found the culprit. I had not had a chance to clean the topsides when it first went back in the water. There were a few small wet leaves plugging the spillover holes on the decksides. The result was the pooling of water that entered the holding tank through the stanchion vent - enough to fill the tank. A minor flaw in the production as if the vent was 2" higher the water could never had reached it and this problem would have been averted. The good news...all it cost me was time...(and a little embarrassment). 