Maybe Darcy did not tell the Col. about the proposal, but he would have had to prepare him for Elizabeth asking about so delicate a subject. The Col. would not want Miss Darcy's improper behavior mentioned anywhere where it did not have to be, and Darcy's trust that the Col. would satisfy any doubts Elizabeth had as to the story must have been based on SOMETHING.

Actually, Elizabeth's belief that Darcy expected she might ask is something I always see as a sign of the times. Modern-day heroines, when faced with an awkward application to a relative stranger will often say, 'uh, no, thanks, er, I believe you,' which really is just what Elizabeth did. We are expected to believe him, too, but if Darcy WAS a villain, he could have been banking on Elizabeth's being too much of a lady to like uncomfortable interviews, or asking questions about embarrassing subjects from anyone involved in the gossip! Remember, she was desperately intrigued with Wickham's history with Darcy, but forbore asking details more than once. Wickham told her anyway, of course, which tells us something about both him and her.