Before Frisk's footsteps fade away, Tauriel might also catch the sound of Chara moving about in their room: They roll away from the door, shakily get to their feet and move to lean against the windowsill and watch the door.

They didn't manage to hear all of the conversation, not with Frisk talking so quietly, but they got the gist: Frisk, for whatever reason, wanted to see them, and Tauriel...

It seems like the woman intends to keep them prisoner.

But unless she also intends to starve Chara to death, the door has to open eventually. Or even if she intends to ask them questions, like the rest of Frisk's... well, friends. So they have to remain vigilant. And ignore the bed that's nearby.