Dust is attracted to electric fields, and sticks to a board in a way that reveals the shape of the electric field in one plane. Dramatically illustrated here on the underside of an Adcom GFA-555. These dusty traces are at only 83V potential, but that’s enough to make them dustier than surrounding traces at lower voltages. Check out how there is a bunch of dust between the grounded mounting bracket and the adjacent trace at 83VDC.