The sensitivity of his hearing here means that with his focus on her, the complete halt of her breathing is all too noticeable, as is the beat of her heart.

His voice quiets as well, once more soft, low, and gentle. "I believe you do." As he speaks, he stands where he is, rather than approaching or otherwise moving, staying still rather than making any movement that may alarm her.

This is something he was not prepared for, but is still telling- he had been concerned that something unrelated was wrong, and while he does not know the source of this it casts everything in a different light.