Board Thread:Naruto Discussions/@comment-24347194-20150211225344/@comment-26302001-20150215114212

Itachi has done his homework on Konoha and all of its shinobi his whole life, he has even researched the Senju clan and Uchiha clan all the way back to before Konoha was made, Hiruzen said so himself. So Hiraishin would not be new to him and he would already know everything about it. Itachi was a studious and curious person. Hiruzen pretty much says Itachi knew about everything.