It says at the beginning of Ch.7 Lydia and Kitty began making daily visits to their Aunt and learnt the officers names and Their lodgings were not long a secret, and at length they began to know the officers themselves'. I guess Mr Denny was one of the officers she encountered at their lodgings. Captain Carter probably did not return from London but if she never mentions him,she's hardly moping! I think JA demonstates how easily other officers succeed in Lydia's affections.