d'Artagnan just pretended he didn't know. Because he figured she'd be happier if she didn't know how worried he was about her. But he had to do something to get her out of the house, away from civilization and the chaotic turn of emotions that went on there.
"Picnic basket with food, blanket... Do we need anything else besides ourselves?" he asked. He also packed the ring he got her, but that was a surprise.