Twenty thousand tons of cosmic dust and micrometeorites fall on earth each year. What's to notice if another few tons of intelligent dust wafts down through the atmosphere to seed rain clouds, give the sunset a pretty hue, and turn everything they touch into copies of themselves?
Most particles would survive the decent through the atmosphere and not heat up any more than if they had been in your kitchen oven.
But the sky did not betray any invasion. The red sunset did not necessarily mark the arrival of the expected plague. The winds did not smell of death. There wasn’t even a pillar of flame descending from the heavens to announce the Armageddon. There was nothing out of the usual.
NASA air quality satellite system, ill maintained since the funding withdrawal, failed to detect anything. No human reported anything unusual. So far, none of the twenty-two billion vigilant nanodefense cells had reported anything either.