I repel because even if resident pair removed, floaters will then be coming by. For me den proximity is not always a reliable indicator of losses. I have not figured out what is a good predictor of losses is other than lack of good repellent capacity.
Simply take a medium to large dog to the den and allow it to sniff around. Fox parents will not like that at all. Response will be that female moves kits to a new location, possibly within hours.