Mike and everyone else knew they were there. Remember Mike offered to feed Johnny's men.
Did Mike see ALL of Johnny's men? When Mike sees the dead assassins lying in the gutter, he is obviously NOT seeing the same men that were in his office that afternoon. Otherwise, he would know Johnny is behind the plot.
Thus, someone within the family must have provided cover for the assassins, etc. Fredo's role could have been to provide these men w/ stealth cover. The assassins lie dead outside Fredo's window; therefore I think that his house is the "command center." I.e-Where the entire operation was being run from.
