You have the gaskets for the fuel level "indicator"; the pick-up tube(s, for multiple engines); sometimes you have access ports for cleaning. All of those can, over time leak -- even it you don't touch them.
If you "Fill" the tank -- you i) add to the pressurization by the hydraulic head; ii) if there is a leak then you're basically relieving it somewhere bad.
It would seem to me that it wasn't just a "little" moisture from a gasket -- but if you had a real failure of the gasket (i.e., 1/2 of it rotted away), then whenever you filled the tank you were spurting a bunch of gas.
Clearly the NOSE is a defensive sensor that should be used. If you smell gasoline inside a boat -- you are really in trouble...as Charlie Stillman pointed-out.
My story of an Atomic IV was from a charter boat decades ago. The engine on the Columbia 34 (as I recall) was sputtering. It would start, then run, then die. We had no wind and had to get back. I sent a guy below to check-out the compartment, just to see if he noticed anything. With a lighted cigarette in his mouth, he opens the companionway steps to access the engine compartment. He shouts up to me "Everytime you crank the engine, gas spurts out from the hose between the fuel pump and the carb". We screamed at him to back up, put the cigarette out in the sink. We turned on the blower and stopped using the engine. After spending 4 hours going 5 miles, we got towed-in by the Coast Guard (back when they helped out.)
My take away:
DON'T TOP OFF YOUR TANKS UNLESS YOU ARE VERY SURE OF YOU TANKS AND HOSES;
USE YOU NOSE, even diesels leak;
USE YOUR BLOWER -- IT'S THERE FOR A REASON; and,
NEVER ASSUME THAT SOMETHING ODD OR DIFFERENT WITH YOU ENGINE DOESN'T INVOLVE A LEAK.