I think him missing is no accident, it sounds to me like fear. Being outside makes a lot of herps nervous. Especially the places we like to put them. For instance in the middle of the lawn or drive way.. on a tree branch..and such, they like to squeeze into tight places to feel secure if not at least under cover. Also I'd lose the hot rock, get a big heat pad. Make sure it's big enough for him to curl his whole body on.