What did those two words 'I know' mean? It meant that Astrid, and most likely even Andreas, knew who they were. But despite that, they still acted like they didn't know them.
His thoughts ran back to the time he first met the twins. During that hostage taking situation when the two saved Nicol. When he first introduced himself, there was a flash of something in the eyes of the two. But that was only for a second, and then their gazes quickly returned to normal. That's why Casey didn't notice anything back then.