In short, exactly what Mutsumi and Miki Kashiwagi discussed remains unknown even to Sakiko, who was eavesdropping; apart from the two people involved, no one else knows.
However, what bystanders could see was that ever since that conversation with Mutsumi, Miki Kashiwagi had been unusually silent—
Not the gloomy kind of silence, but rather the casual kind of silence one has when stumped by a minor side quest while playing a video game.
Chapter 162