An asteroid-mass black hole is around a micron across. It's not going to be nomming on much because the matter distribution inside the solar system isn't that dense.

Any tiny black hole born in the big bang would either have evaporated (if Hawking was right...) or would have grown much larger by now.

Even a moon-mass black hole (0.1mm) wouldn't be eating much, although its gravitational effects would be much more obvious.