I explored the problem further....
I took out all the interior of the trunk, and I found the same gasoline deposits on top of the right wheelwell (white with brown dots) as I found on the inside of the gasoline filler tube (right by that wheelwell). Seems to me that this has happened intermittently on this car over the years. I checked for leaks and found none (if the gas tank depressurizes with a hiss as you remove the gas cap, you KNOW you don't have a leak, because the air used to pressurize was not able to escape until you removed the cap!)
Temperatures change rapidly here in Washington, D.C. I now never top off anymore, and have had no recurrence of the problem. So I guess the rule is once the pump stops, you stop fueling.
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)