Its really neat but have no idea what it is. I figured that I would post it. I think it may be a part from a piece of farm equipment? I took a brass brush to the bottom and can just about read a patent on it.
Well, the one you have may have decided to part company with some signage or other equipment that was once there, perhaps just for a time, and it simply wasn't convincing enough to its fellows for ALL of them to join him in making a break for it. I believe some railway equipment also used such reflectors, in case there was an old railroad right of way near where you found it.
By the way, below are a couple-few of shots of a old hazard sign using what appears to be those same doodads or very similar. A bunch more related items are also found on FleaBay, where they're also known as "cat's eye reflectors", in reference to the fancier marbles of, say, Huckleberry Finn or Tom Sawyer's youth...
See the missing one from the "2"? That's how you found just one.