When Draco is upset, or is preparing to tell Harry something that he won't like—well, he almost always folds socks. There is something calming about the activity, Harry guesses, that gives him the focus to say to Harry what he wouldn't be able to say without the necessary distraction.
It's annoying, and mystifying, but oddly endearing just because it's Draco.
So Harry isn't exactly shocked to see that Draco is folding socks again on a rainy afternoon like any other, except it wasn't quite a normal enough afternoon—because Draco has that Look, and his suitcases are out; and besides, Draco's home from work rather early.
And it comes as no surprise to him when Draco says, carefully looking at the socks he folds into his suitcase, "I can't come back."