When you don't keep your tank full, moisture condenses inside on the
cool tank sides as the atmosphere heats up and cools down. Your tank
"breathes" through the vent. That's one way, at least. Check to make
sure your vent isn't turned upward where it can catch rain. I bought a
Cruisers, Inc. boat that came from the factory that way! If you can't
find obvious leaks, just keep the tank full and that will probably
solve your problem.