Make sure the gearbox oil dipstick is tightened properly and the small bolt that is right underneath the transmission case u have to get right under the bike to see it
well if its from the rear wheel hub and you have managed to kill a seal in one ride, i would replace the bearing and seal. how much oil is there? there sould not be loads that can even get to that seal. have you over filled the gear box with oil. there is a breath that oil can get out of if its over filled.
just check all the bolts, and the one thats underneath the trans case, BECAUSE when i opened that on my Piaggio NRG it leaks out the gearbox oil and it does hit the wheel. So check all the seals and bolts then it should be ok