Wipe her butt?

radix023

So I think I resolved this. The link I had found and related previously from the Mercedes forum seems to be the one. I had some black gunk under the filler flap that I thought was just oil or dirt from spill from the prior owner. After reviewing the Mercedes article and not getting traction from the community here, I hypothesized this was mold. So I used a bleach solution (Tilex) and an old sock and sprayed down the area under the filler flap and wiped out all the black gunk as best I could without taking things apart. It seems to have resolved the issue.