If the holes are only about an inch across then it is possibly mice. If the holes are of irregular size and shape then the soil has slumped to fill void spaces below. For the latter, mix up some dry gritty compost and feed it into the holes, finaly tamping it in firmly. I get this regularly.

Or shrews. We get a lot of nesting shrews/voles in our rockeries. Mice are a nuisance, they eat crocus bulbs. If it is not slumping then try putting a few pieces of rat poison as far down the holes as possible (stops birds getting them. These will kil mice. but not the harmless (useful insectivores) shrews or voles.

if they are about 1cm then they could be soiltary bees too.