i used to use a locator when i 1st started ferreting. now i don't bother. i have ferrets that don't lay up and will come to the whistle. it's easy to train your ferrets to come back to the whistle if they don't lay up, just whistle before u feed them then if one goes missing in the field a quick few whistles and your ferret should soon come bounding back.
works for me anyway and have never lost a ferret yet.
keeping quite is the biggest key to stop your ferrets laying up, standing down wind and out of sight also helps too.