They're present in my compost/mulch pile and I didn't put any in there.

My compost piles were always well populated with red worms, so there must be some good corrolation.

Actually, my DD had a red worm bin and they fed it vegetable scrapps.

The love to feed where the compost material is in the process of decomposing. However, they do not "turn it in to vermicompost" - (which, let's be blunt, is just worm poop). They just leave their droppings behind in the compost. So, in other words, they are adding to, not replacing the compost.