The Masqueraders by Georgette Heyer

Robin preserved the light manner, but he had begun to chafe at his petticoats. Faith, the old gentleman seemed to do nothing and there were rumours current now that Rensley, as soon as he was able to leave his room, meant to bring a case against his would-be cousin. Robin had small mind to go on playing the lady indefinitely. He believed the Black Domino remained in Letty's memory, but he had little chance of seeing her as the days passed. She was out driving, or she was visiting, or even she was indisposed. When he did meet her she was abstracted, and volunteered no confidences. There were shadows under her eyes: her aunt said it was no wonder, since nowadays she was seldom in bed before midnight; Robin dared to hope a Black Domino had induced this wistfulness.

Prudence thought nothing at all of it; she was rather preoccupied with her own affairs, and showed but slight interest even when Robin spoke of John's new behaviour. Robin became aware of the frequent absences of his faithful henchman, and receiving only evasive replies to a sharp question or two, immediately suspected activity on the part of my Lord Barham. Prudence said placidly that it was very possible she thought they were like to know all soon enough.