I bet some air will trapped in the tank that way. Also the temp will have to stay the same the whole time you are testing.

They make a dye that you can put in your fuel that is traced with a simple UV flashlight. I don't know if it'll work for your situation but it made it easy to spot a single drop of oil from yards away. $10 at any auto parts store.

I had a similar issue on my Pantera. I had owned it for 3-4 years. First I checked the vent hoses. Then I pulled the entire cockpit interior and floor to inspect the tank.
Couldn't find anything. Finally, I checked the fuel fill. The clamps that held the hose to fuel fill were both loose as chit and the hose had loosened up with all the vibration over the years that fumes were getting into the cabin.

I agree, it happened to me on my 1992 Checkmate about 5 years ago. Noticed the fumes and then small amounts of fuel dripping from the drain plug hole while on the trailer. Luckily it was winter and I had time to replace it. The tank was unfinished aluminum and badly corroded on the bottom where it was foamed in. Not a fun job to replace.

One other thing. Two years after changing the tank I noticed fumes again in the bilge, especially after filling up. It turned out ethanol destroyed the gasket for the fuel sending unit on top of the new tank. That was an easy fix and something else for you to check.