Mystery

A lovely gardening day. I keep my composted leaves in a separate container from the garden compost and some of it was ready to bag up, so that I could make room for this year's leaves. Perfect day to do it. I hadn't gone far when this egg emerged. The leaf mold is about 3 years old and there have been no chickens or ducks in the neighbourhood for at least 5 years, so this is a real mystery. It's in a conventionally sized egg cup to show the size. Any ideas?