How long b4 property left on your property is considered abandonded?

We have been using the propane tank on our property since we bought this house. Over 9 years now. Suddenly out of the blue and propane company comes to my door and says the tank is theres and they want it back. What are my rights here? I have nad no contact with this company in the nine years ive lived here and was under the impression the tank was owned by the previous owner.