Do a good clean job on it. Next time you go riding, try to isolate exactly where it comes from. Overtightening can be worse than a small drip so be careful about tightening things. I've found that chain lube can work its way down and appear to be leaks as several places. Maybe take off the sprocket cover and clean up very well. If the output shaft seal was leaking and has been replaced, it could have been damaged on installation so don't rule it out.
