Rumors? From who? Don't forget that even Sasuke didn't knew Susanoo when he saw it the first time, and he did read Uchiha tablett tooCause he knows everything lol jk.
I'm guessing he heard rumors about it.
Lol who is tickling you?lolllllll he herd rumors about it ... kakashi is a smart man he can put two and two together lmaooooooo