[Technically anyone is welcome to go through the Investigation Team files but Ianto also doesn't exactly trust people not to destroy his system of organization. So when he spots someone doing just that he heads over to be friendly and helpful and surreptitiously put things back where they're supposed to be. It's only when he gets closer that he realizes just who it is. It's not as though Reid isn't allowed to be there or that he doesn't trust him with files but it is a little surprising. If it were anyone else he might even categorize it as suspicious.]