I agree on the ferret moving on as soon as the rabbit is killed, all mine do, there's more interesting things for them to do than sit around eating and sleeping, once everything is dead or bolted, out they come. (unless blocked in like last weekend, would have been a lost ferret but for the locater.)
If the batteries die then use your own senses and the dogs to put you in the picture, this has happened on collar batteries occasionally, or where they don't have a good contact. Either dropping in another ferret equipped with collar to find the other, or finish the job off and come out. (dependi