I agree in this case that you have invited trouble by neglecting the hives and by not keeping in touch with the landowner. At the very least now you should pin a polite card with your contact details under the roof of each hive, and photograph them in situ, but that won't stop them walking. Are they marked to aid identification? Difficult to just shift them if the bees were introduced rather than a swarm - but how to prove this either way though?
Probably an innocent misunderstanding, but beware assuming that this is the case. Plymouth has been plagued for twenty years by an individual who steals hives by claiming they are neglected, telling the landowner that they are a liability as such and need to be sorted out, then removing them for "safe care".
These are not neglected hives; in the three instances that I know of, the hives were regularly inspected but whole apiaries were suddenly cleared out in this fashion. When the landlord is a large organisation, it is common for estate managers to come and go over the years, and for beekeepers' details to be lost. Somebody coming in offering to help sort out a 'problem' can understandably be welcomed by a new manager.
The police were not interested as they claimed it was a civil matter, and Devon Beekeepers Association were not interested because they said it was a matter between individuals; the suspect individual also held a County-level role in DBKA for many years, read into that what you will.
Assume that the worst will happen you will be left on your own to find and repossess the equipment if it disappears.
The BBKA produces a leaflet entitled "Abandoned Hives" which provides further guidance on how to deal with apparently forgotten hives/colonies.