"So he balances your books." Now she does sound skeptical. Natasha doesn't know Eames's work, but that seems unlikely. And while she doesn't have a problem with secrecy, this is getting frustrating.

She turns her attention to Arthur instead; at least he doesn't find the situation any funnier than she does.

"You know what he is," she says, not quite asking a question. That seems like a good start. It's vague enough that if she's wrong, she can still walk this back—but she sincerely doubts she's wrong.