In chapter 11, we find that Darcy is "awake to the novelty of attention in that quarter" when Carloine asks Elizabeth to join her in taking a refreshing turn about the room. (He recognizes that it's a bit odd for Caroline to be showing Lizzy any attention in this way.) We already know that Darcy is aware of Caroline's jealousy and dislike of Elizabeth. We also know that the only reason why Caroline asks Lizzy to join her is because she hasn't been successful in getting Darcy's attention focused on her "elegant figure":

[Miss Bingley] got up and walked about the room. Her figure was elegant, and she walked well; but Darcy, at whom it was all aimed, was still inflexibly studious. In the desperation of her feelings, she resolved on one effort more, and turning to Elizabeth, said --

"Miss Eliza Bennet, let me persuade you to follow my example, and take a turn about the room. I assure you it is very refreshing after sitting so long in one attitude."

"You either chuse this method of passing the evening because you are in each other's confidence, and have secret affairs to discuss, or because you are conscious that your figures appear to the greatest advantage in walking; -- if the first, I should be completely in your way, and if the second, I can admire you much better as I sit by the fire."