Missile is above average intelligence. That is not the same thing as flawless comprehension of human tongues. He can pick up words like detective with repetition, but he does not innately understand them. That is because the languages of Babel are born from human pride of a kind he is lacking.

He also may not have understood why Levi claimed detective, or that he ought to be suspicious of it. But he came to understand and make himself known. Incidentally, those pictures were from the paper which I had shown him.