I get a greasy brown mark appear on the left side of the frame sometimes, I guess because the stand leans that side. I hope it's just toad being washed out or something but I suspect it will be something catastrophic. Also, if I turn my fuel tap to the off position, fuel pisses out!?
But anyway, can you not trace the trail to it's source? If it's too dirty - clean the whole area and wait for it to happen again. To be honest, this is pretty terrible advice as something could fail whilst waiting for it to happen again!
I'm going to overhaul my bike and try to cure its many ailments this weekend. I'll let you know if/when I find the source of my 'stain'.
Don't these bikes remind you of the good old RS?
![:smt002](./images/smilies/002.gif)