The morning after a long desert ride, I noticed a small pool of gear oil from the rear transmission, just under the drain plug.  After investigating, I found the transmission oil dipstick was missing its o-ring.  The poor seal allowed for a leak.  Not a big deal, and easy to replace the o-ring, but others might want to check theirs to make sure they have one.  The engine oil dipstick had its o-ring, just the transmission oil was missing one.