Aha - a related thread from Renntech on this issue with a possible theory:
http://www.renntech.org/forums/topic/23470-grease-coming-out-hole-in-rear-axle/
Sounds like the left side got hot one time and some grease vented out the vent hole. So I guess I need to get under there and check my boots to make sure they have enough grease?
Left rear tire makes kind of sense - I have TC and I was at Roebling in October when I think this happened and had 6 hot sessions that weekend, and picked up a lot of speed, taking 2.1 seconds off my previous fastest laps. I bet the TC was going off and braking that left rear wheel and heating it up?
One of these days I am going to turn off that TC... but I am not there yet... Every time I turn it off at autocross it seems like I spin...