Usually when claiming adverse possession of land, the squatter has to physically openly fence off the land in question and then claim adverse possession after 12 years has elapsed. Courts will usually only entertain adverse possession claims after a period of 20 or so years has elapsed.
If the container has been there forever, the vendor would have to disclose this and the nature of the arrangement as part of the sale. Nonetheless, the container owners may have alternative or accompanying claims in addition to adverse possession eg claim of right of way, licence to use the land to store the container, prescription for long use etc.
Proceed with caution as you may well decide to go ahead and purchase, only to have an awkward discussion with the container owners and potential court proceedings after. This is something the vendors would need to sort before you think about purchasing.