This last summer I noticed a not small amount of diesel in my bilge. I've looked under the engine and found nothing. Also put some paper towel on under the fuel tank, left it for several hours, and detected nothing on it when I pulled it out. The only thing I can think of is that at the end of the previous season I had a dockhand fill the tank and she put in way more than I expected, considering how little I'd used the engine that season. 

Is it possible that the tank was overfilled and fuel leaked into the boat via the vent?