I agree that CW had tried his best to get info about Anne without her knowing it. I think it was more than curiosity. I think he was trying to ascertain for himself if his own feelings for her were still there, and what she'd been doing the last 8 years.

I think he was a bit taken aback when Louisa told him that Charles had proposed to Anne. I'm going out on a limb here, but IMO, CW got along famously with Charles - they went shooting, they went hunting, and CW spent a lot of time in Charles' parents' house. I think the two men hit it off from the get go. So to hear Louisa tell him that Anne had turned down such a good man, why, that made him wonder. That would explain his pause upon hearing the old news: