My last (in a previous life) sailboat had the smell of diesel when I made the purchase, thinking the PO had a mechanic in and maybe he spilled a little diesel. Five months later I am still chasing the smell, then the red bilge appeared, diesel in the bilge. Pulled the 34 gallon tank, found a hole the size of my thumb in the bottom. Had two 17 gallon tanks fab'd and installed them. All is good (I think), one week later, I have the red bilge again and after removing the only fuel tank a filled I found another hold in the new tank, in the same place as the original tanks failure. What I found was a screw below the surface of the platform the tank sat on that was spiking into the tank. I beat the H3LL out of the screw, layed rubber strips on the platform and repaired the now two week old tank, then re-installed it.
Since the hole was on the bottom of the tank, it would only leak at different times, not a steady flow. The repaired tank is still in use, now I have a trawler that came with a live aboard raccoon, but that will follow later.
Absolute,
"Suckin Sludge & Havin a Gas"