Well the part shown by David keeps the timing chain from slapping around in the cover and places light tension on the chain. You state that you were working on the transmission and so would presumably rule out that particular part. Again looks like it mounts into the transfer case housing but I am not sure as I cannot find a clear picture or diagram.
However, if you were not in the transfer case that would rule out that possibility too.
Is it possible that it was just laying around and not a part from what you were doing that day?