No one noticed the rock.

And for a very good reason. The rock was nondescript, one of millions of chunks of rock and ice floating in the parabolic orbit of a long-dead short-period comet, looking just like any chunk of that deceased comet might. The rock was smaller than some, larger than others, but on a distribution scale there was nothing to distinguish it one way or another. On the almost unfathomably small chance that the rock was spotted by a planetary defense grid, a cursory examination would show the rock to be composed of silicates and some ores. Which is to say: a rock, not nearly large enough to cause any real damage.

This was an academic matter for the planet currently intersecting the path of the rock and several thousand of its brethren; it had no planetary defense grid. It did, however, have a gravity well, into which the rock fell, along with those many brethren. Together they would form a meteor shower, as so many chunks of ice and rock did each time the planet intersected the comet's orbit, once per planetary revolution. No intelligent creature stood on the surface of this bitterly cold planet, but if one had it could have looked up and seen the pretty streaks and smears of these little chunks of matter as they burned in the atmosphere, superheated by the friction of air against rock.

The vast majority of these newly minted meteors would vaporize in the atmosphere, their matter transmuted during their incandescent fall from a discrete and solid clump to a long smudge of microscopic particles. These would remain in the atmosphere indefinitely, until they became the nuclei of water droplets, and the sheer mass of the water dragged them to the ground as rain (or, more likely given the nature of the planet, snow).