I'm in the group that believe the OP, that it was stolen. We almost had a propane tank stolen from our pop-up in a town called, Normal, Illinois, many years ago. Stopped at a restaurant right down town, "Normal" and when we came out, the cover of the tank was pulled off, and the hose unhooked. They couldn't get the tank off though because of the way it was it hooked in. I even had to fight the thing to get the tank off, and I knew how to do it! Reported it to the local police who said it was not uncommon for RV to have propane tanks stolen for meth labs. We were luck they didn't get it. So you know ... I believe the OP that it was theft.
When we purchased our Outback, the first thing I did was remove the tire from the trailer bumper. Our bumper came factory equipped with a hitch, so the bumper is welded very well, and welded the hitch frame and the bumper together. So I felt very comfortable taking the tire off and putting this (below) on. That meant the spare tire now rides in the back of the pick up truck. With all the stuff we carry in that pick-up truck bed, it would take a lot for someone to take the tire!