Were there more than just two mirrors? The first time she witnessed Antonin use his, he wasn't bothered by Hermione learning that he was in contact with his best friend. It bothered her when she stopped to consider the moment they were still laying on the sofa in the cottage. Before whomever was contacting Antonin could even speak, he'd closed the mirror. That told her that whoever was trying to contact him was someone Hermione knew and a voice she would recognize. Clearly, her husband didn't want her to know what was happening. Just one more example in an ocean of them why she needed to move on. As safe as she felt in his arms, it was worthless if he couldn't trust her.