I don't have outdoor bins, but years ago had large bins in the basement and used the castings (which contained cocoons) to fertilize plants. Years later, I notice redworms in the darnest places. Pumpkins laying alongside the garden for a long period, produce redworms underneath. Leaves decaying down by the creek have redworms. I believe I inadvertantly introduced a small population all those years ago that has somehow survived. Now having said this, I have yet to find any large population invading my compost, but my finished compost doesn't stay in a pile very long!