No ideas on how they find the hives, but if I were you I would be looking into fitting the hives with some sort of tracking devices much like this site offers
http://tlrsystems.com/property-recovery.html although that is a US site, there must be someone doing the same in UK. You would also need to look into the legal side of using tracking devices.
Thanks, that is good thinking. I did see a recent Click programme and some of those devices look pretty good. I think all the pieces to the hives would have to be security tagged too - could end up costing a fortune :(