He had indeed heard them approaching, and had hidden his increasing tension from her, but once they came near enough that even she could hear them, and she moved back to the library/entrance area, he didn't have to conceal anything. He was geared up for slaughter when, just before the booming knock, he recognized one of the voices, and his face fell.
Faster than thought, he went to her, moving so quickly he seemingly appeared next to her as if by magic. Clapping a gentle but firm hand across her mouth to keep her silent, he wound his other arm around her waist and flashed them both into her bedroom. The transition would be dazzlingly quick for her, so he put her back down propped against her new bed to recover while he closed the door and returned to her, whispering urgently directly into her ear, so that the creatures out in the tunnel wouldn't be able to hear.
"Charis, these are fellow monsters, come for a social call. They have not softened toward you, as I have, and in groups, they tend to be even more vicious. I don't want to let them at you, but I can't deny them a brief visit -- our society is rarefied enough, I'm required to play host. I want you to stay here in safety; they'll be able to hear and smell you, but they'll think you're locked up. I'll leave the door ajar, though, so you'll be able to listen to the conversation. Will you do this for me?"