Cosette's eyes close for a moment, and her lips press together in a smile that's halfway to tears once again. It makes no sense that that small, private gesture of tenderness should undo her so -- and yet there it is.

She lifts their joined hands to press his hand to her pale cheek. "My Marius. I do love you."

After a moment, she smiles at him again, a little more composedly. "I'll see you in just a little while."

And her mother's room is just there. She'll just let herself in; she has a key. If Fantine isn't there, she'll... well, she'll figure the next step out from there. (But she might spend a few minutes letting her self-control fall in private, first.)