Holes in the lawn

I'd like to know what is causing these holes in my lawn. Diameter approximately 3 cm, although they grow over time. Blocking the holes leads to them reappearing about 2 days later, usually during the day.. A trail cam on the holes has shown no activity near them over the last two months, but the holes keep reappearing. However the cam has recorded the interest of the local cat who has plunged a paw down more than one of them.