Mysterious Diesel Leaksk, Multiple Tanks at the Same Time

I filled all six 200 gal tanks through the four fill openings (two each side, one tank daisy-chained for fill) then departed on a multi-day cruise. One leg was particularly boisterous. When I returned to home marina a week later I smelled diesel in the bilge. On inspection, the areas below both the port and starboard fuel tanks showed small puddles of fuel. I checked that the tanks hadn't become pressurized with the rough seas. I looked for signs of overfilling and subsequent spilling through the vents--no signs. I soaked up the spills and went home. Two weeks later the leaks were continuing, small but smelly puddles in the same places as before. I checked again for pressurized tanks butt it was not the case. I haven't changed the supply/return valves since installing new aluminum (5250) tanks, valves, hoses about 18 years ago. All this started with my filling up all tanks.

Before I pull all the panels down to take a visual on the tops and sides of the tanks I'm looking for your ideas. It is unlikely that the guage sending units on both sides failed at the same time. All input more than welcome because right now it is a real mystery.

Did the temperature go up and the fuel expand? Was it unusually cold when you filled? Not entirely unusual that a fuel sender is not entirely fuel tight and expanding fuel would answer the coincidence of both tanks at the same time if you filled them equally full.

Use some of the Scot paper shop towels, the blue ones, and wrap them around ALL connections.
They will show a distinct colour change when wetted. May help determine source.