Nanodust
Layers of dead nanomachinery sedimenting in the dust, on the bottom of lakes and elsewhere. Ideally nanodust should self-destruct, but there are always bugs in that. Micro- and biotech scavengers collect them, but there are always places they miss. So the puddles on a roof collect diamond dust which blows away when they dry out, which can both erode shiny facades, irritate people and catch fire.
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