A meth ring is a possibility
When I lived in Washington state, there was a local that blew himself up, and the property was littered with all sorts of propane tanks. Everybody was publicly speculating meth, because he lived in an area known for drug trafficking. Turned out he had converted a car to run on hydrogen, and had blown himself up trying to fill a propane tank from his homemade hydrogen generation plant.
Meth cookers use propane tanks to steal anyhdrous ammonia. There is a side effect in that the brass valves turn blue. The color is a tipoff. That would be a lot of Anhydrous to steal and haul around, and one small leak would make the thief pretty miserable, if it didn’t kill them.