What is the explanation of the apparent staining inside this airliner window [photo]? On a recent flight, I noticed a pattern of staining or dust particles inside the window, on the plastic reveal within the cavity.

It looked as though a long-term and consistent pressure differential between the cabin and the cavity had drawn a stream of air into the cavity, resulting in what looked like a sooty stain.

And even more oddly, as you can see in the picture, right opposite the vent, the mark is very concentrated, as though formed by a high-pressure stream - which I presume isn't actually the case.
I believe the stain is caused by air going through the hole relatively fast due to compression of the cabin. Assuming the window is 40x40 cm large, and there is 10 cm between the panels, the volume of air in it is 16.000.000 mm3. About 30% of this air comes out due to the lower pressure at altitude, that is 4.800.000 mm3. Assuming it takes 600 seconds (10 min) to pressurize the space again, the flow will be 800 mm3/s. Assuming the hole has a diameter of 2 mm, it has an area of 3.14 mm2. The average flow velocity is then 254 mm/s. It looks like the spot is only 20 mm away from the hole, so it would take a dirt particle less than 0,1 second to get to the spot from the hole. I think that is not much time to deflect, hence the concentration in the middle.