I'll make a little note. But I do enjoy the chat roulette style mystery of it all!
Living on a dirt road, one does aero testing every day. Just like Basjoos said, the dirt/grime deposits where flow is detached. I find little lines of dirt on the trailing edges of panel overlaps, and a lot on the very rear panel.
The good news is the boat tail on the whole is very clean. The bad? About 6 inches from the end, on the bottom, I see wisps of dust accumulating. I think I'm losing attached flow at this point, just inches from the intended detachment area. Tuft testing will come soon- when I can find a willing victi- er,
volunteer. That should diagnose the deposits. If it's bad I will have to lower that surface just a bit.