I have had this story in my head for the past few days, but for the life of my I can’t seem to identify it!
The setting is a gentleman’s dinner club, which requires its members to wear green jackets due to an unfortunate incident some years earlier. A thief had made off with all their precious silverware, bit by bit, by posing as a guest and a waiter simultaneously. The ruse was only discovered when a detective was installed in the building, who deduced from the different treads of the thief that he was playing two characters - loud and brash as a guest, and tentative and quiet as a waiter.
I was sure that this was a Sherlock Holmes story (the distinct way in which I remember the story being told in the past tense would lend itself to that), but I have read damn near all the wikipedia synopses of Conan Doyle’s short stories and it doesn’t seem to be among them.
Right- the thief Flambeau had figured out that, at this club, the waiters AND the aristocratic members BOTH wore formal suits. Which meant that, depending on the way you carried yourself, you could pass for either a gentleman OR a servant without changing clothes.
When Flambeau was among servants, he adopted the posture and gait of a gentleman; when he was among gentlemen, he took on the posture and gait of a servant. That way, neither the real servants nor the real gentlemen would look at Flambeau and think, “Hey, who’s that? We don’t recognize him! What’s he doing here?”
Because, after all, a gentleman at a ritzy club hardly pays attention to the servants around him, and a servant wouldn’t think twice about a new, unfamiliar millionaire in a tux at the club.
Father Brown was sitting in a downstairs hallway, and caught on that SOMEBODY in the hallway above him was alternately walking briskly (like a servant rushing to bring drinks to one of the rich club members) in one direction and then casually in the other direction (like a pompous aristocrat ambling noplace special).
Brown figured out that someone upstairs was pretending to be a servant OR a gentleman, depending on who was passing him by in the hall. Flambeau didn’t even need a disguise- since everyone was dressed the same, he could pass for a milionaire OR a waiter, just by adopting the right mannerisms.