Those grey boxes formed behind the engine cover is probably
dust. Dust is usually attracted there due to the heat on the engine
cover. A surface with high heat tends to attract dust or sometimes,
carbon molecules. Over time this forms a layer of "dust".
The shapes of boxes you see is the shape of the frame of the
rear cover. The lines werent covered in dust is because this areas
have lower temperature (which was insulated by the frame), than its
surroundings (those without the framework behind it).
Actually more likely static emitted by the engine attracted the
dust, and more likely those are carbon, not sometimes, because of
the close proximity of the exhaust fumes. The carbon molecules are
usually polar by nature. It's like the experiment where water
dripping tends to veer towards a electrical wire with a current
flowing through.
And the shape of the "dust" is not that "scientific" either. To
put it simply, if you're attracted by something you would travel
the shortest route right? So if you're blocked by a frame, you
would just get through the vents and go straight right?
