Cain had purposely spread his Ego Wave through open fields where Wave Beast would be present and avoided the flank of the mountain range that was perfect for hiding. He did this to generate the impression that he had not noticed anyone when, in reality, his [Scan Force Field] had already detected the presence of the assassins and even showed him their strength.
Although Cain could immediately hide his killing intent and coldness, not blowing away his cover, he still frowned. The fact there were assassins at the beginning of their journey meant they were here before they arrived.
Shura's destination for the Purity of Body should be a secret, yet assassins were already waiting for them, so somebody leaked the information.
Cain did not bother with the internal power struggle of Blood Sea and focused his mind on the task at hand.