While Darcy does not tell us what arts where used in keeping Bingley unaware of Jane's presence in London, we do know that Jane's letter to Caroline informing her of her London stay "had by some accident been lost." (Chapter 26) Did Darcy somehow gain access to the Hursts mail or was he in collusion with Caroline and Louisa? Or, perhaps it was really lost? Don't think this suits JA's purpose - fits Jane's willingness to find an acceptable outcome to the missing letter without thinking ill of anyone. I also find it hard to believe that Darcy would arrange to intercept someone else's (the Hursts) mail. I rather think it was more likely of Caroline and Louisa.

The passage when Jane called at Grosvenor Street unannounced because Aunt Gardiner was "going .. into that part of the town", found Caroline at home, though not in spirits (!), was therefore a "lucky break" on Caroline's side and unfortunate on Jane's that Bingley was out that day. Was Caroline's reproach to Jane for not giving her any notice of coming to London genuine or is she flat out lying to cover her ignoring the communication and not responding? If Caroline was indeed being deceptive, she may have thought she was earning kudos with Darcy for aiding him in keeping Bingley ignorant of Jane's presence. Methinks it more probable that Caroline was deceptive than Darcy meddling with the mail.