* Campion's reaction/quip to Lugg (and attending policemen) upon finding a murdered woman on his bed after coming home from a mission:
-->"Is the lady in my bedroom staying long?"
* Several of Campion's introductions into the story in the earlier books, like his obsession with a pet mouse in "Mystery Mile" or dressing up in [[Franchise/SherlockHolmes a deerstalker]] to impress a client.