Chip you are jumping to conclusions way too quickly. maybe just maybe your neighbor has simply borrowed your gas tank? leave the gate open at night so he can slip back in and reinstall the tank again for you. Maybe he thought you would be out of town so you wouldn't be needing the thing. When you do catch him connecting the tank back to the line, just laugh it off and offer him a beer.
Good luck
AZB
Good luck
AZB