I will take a look on my 07 MMH. I also remember somewhat the same thing on my former 03 Navigator (only saw it by standing on the bumper to wash the roof). It didnt seem to matter asthetically or had any problems, so didnt think much about it.

If you dont see it, and it wont cause any premature rusting (not likely here in So.Cal) or chipping, or something, I would probably not worry about it.