True enough but registration is not a great imposition and is a big help, not intrusive as they just want to know what hives are where. It literally asks your name and address, e-mail and how many hives you have, that's it. Helps the government know how many apiaries are in the area and the number of keepers. Helps track colony numbers nationwide and any changes.
You don't have to register but if you don't and you have a problem with neighbours or damage then this is a big help. If registered and someone on your property is stung or your hives are damaged you can claim on insurance, if not you can't so I thought it best. Each september you just go online, update hive numbers if necessary and hit renew.
For unregistered abandoned hives there is nothing that can be done, they can't make you register, but if the registrtation number is shown on the hives any problems and they know who to contact. The number can just be written on any one of the hives in an apiary, all in all a good thing and I was happy to do it. Any problems in an area and they can contact all people to let them know, who would't want that.
I still think for the OP's possibly abandoned hives just a note in a plastic bag should do it. If after a month or so this time of year it is still there and no response it is safe to say no owner. Left till the spring, as shutting down now, pretty much a guarantee.