I sent J down with a flashlight to check the barn when he got home yesterday. No sign of whatever-it-is. He put some paper and such at strategic locations to see if anything got moved. Nothing was moved when he went down to put the girls to bed. Nothing was moved when he went down this morning. Lid still firmly on can.
So much for whatever-it-is being nocturnal. Raccoons are very nocturnal. Clearly, it's a daylight poltergeist that is messing with my head. Joke's on whatever-it-is. There's not much there to mess with.