Answer: Sometimes an animal will dig in fresh mulch, with cats using it as a litter box being primary suspects. Squirrels and dogs will both bury and dig in mulch and I suppose a turtle might dig into mulch when it is time to hibernate as well. Yet another possibility is toads whose activity you might see in the evening, especially after watering when the area is moist and slugs (a favorite toad food) might be out. Perhaps with a little careful observation you will be able to identify the culprit.
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