Was there an intact "bottom" to the drum, too?
My guess is the drum was used to stash something. That square opening on the top looks like a good place to stick a 2x4 to pry the top open quickly.
You might want to get a metal detector, and sweep the area for more metal. That "witches circle" was definitely used for something shady. If not moonshine, they may have been cooking meth or something like that. Keeping it overgrown and hidden would be a definite advantage.