There is a weep hole in the swingarm where oil can drip out. On newer bikes, there was a slew of reports of "leaks" that turned out to just be grease from the shaft getting hot, running down inside and dripping out the weep hole. No problem and went away after a while. On well ridden bikes, there have been a few seal failures in the diff pumpkin at the shaft that can allow oil to leak out and run thru that same weep hole. If memory serves, there are two seals that have had issues. One was clearly a scratch in the mating area that damaged the seal when it was pressed into place. I don't recall if the others were just wear over time or something else. The shaft is retained by a circlip of sorts that has to be removed before the shaft and seals can come out. The parts were not expensive.
Other possibilities are simply a dry seal from lack of use. A few rides might clear that up. ;D Reports of oil from the inner cush drive seal have also occurred, but I don't recall the outcome.
