What's That?

I was working outside when I caught a glimpse of someone in blue jeans and blue shirt rushing by. By the time I looked up, he was gone out of sight past the garage. He was coming from the yard or porch next door across the alley from my house. When I looked back in the direction from where he came, I noticed a jar sitting on the neighbor's porch. It had liquid in it. I didn't feel like I should go up into their yard to see what it was, but the curiosity has been killing me. Why would a guy come up the alley, sit a jar of clear liquid on someone's porch, and then rush off very fast? What could it be? Why should I care? But I guess that's the nature of humans to be curious.