We discovered that a very nice but dimwitted chap in the terraced street one across from us was live-trapping the mice in his house and releasing them "to the wild" in the park. And as they were house mice and not park mice, they became an easy target for Flynn who promptly brought them home. If we got live ones, John used to take them to school and release them up there where they could choose between buildings and the great outdoors. Heaven alone knows what got them next.