I'd say it originated at the valve cover gaskets from the wind pattern. No way to tell with the motor that dirty. Clean it up with a pressure washer, get it spotlees then run it in place checking for the actual source.
That's your crankcase breather hose and it vents into the airbox. Possibly a dirty air filter would draw more vacuum out of that hose and suck in more oil. Possibly you have excessive blow-by causing it to vent more than usual. Take off the tube, clean it up, clean up the intake box and see how long it takes to come back.
On my second look, is the end of the hose even hooked up to anything or just a screen over it???