I am so dumb! I didn't pay attention when Yuuji-sama mentioned it because, well, they say he sells butlers to the circus but what if he sold Daisuke to the circus!!
It would explain why no clues can be found, because he's certainly rich enough to hide his evil deed and all that goody-two-shoeness really hides a very, very black heart.
The more I think about it the more it makes sense.
Oishi Shuichiro-sama sold my cousin to the circus!!!
He broke away from his habit of selling butlers and started selling off the students!
OMG! What if he's plotting selling another master to them??? I'd better put alarms in Yuuji-sama's rooms when I go back!