Oil tank leak most likely. My 10 Roadster has had the same problem. Noticed it after about 5k miles on it... new bike for petes sake! I bought the o-rings from Triumph but never installed them. Loosened the tank, wiggle it, tightened back down seemed to have done the trick. I will however get a little bit oil now and then. Seems to be a engineered defect if you ask me.