"Smoke" grime on trunk rear?

[Zerohour3k]

Touchy thread, since it could fall into either detailing, or here.. but only diesels would have this problem.. so..

Ever since I bought this car (1984 Euro 300D), I've noticed (over time) accumulation of black "spots" on the rear of the trunk. This was a heck of a lot more noticeable when the car was white.. however, I repainted it to black (after a mishap at a gas station.. free paint job, but that's another story).

While the car is black now (and thus, hides a lot of the spots).. I see them there again, after a while. Note that these DO come off, with some heavy elbow-grease.. and a good scrubbing.. but it makes me wonder if this is due to a problem of some sort.

Now, it's no lie that in cold weather (which provides great cooling for these straight 5's).. I like to give her the ol' Italian Tune-up.. and the car IS a little louder than the US models I've had (probably due to lack of insulation behind the dash, like the US model had). No blow-by in the car, and only a small oil leak from the filter housing. Car appears to be in tip-top shape (except an elusive water leak on the interior). My question to you guys? What would cause this.. (or is it normal..?).. and what is the best way to clean this off (without damaging the paint). Thanks!