Just read The Adventure of the Circular Room in The Game Is Afoot collection my husband gave me. It’s an August Derleth story that he apparently first wrote as a Sherlock Holmes pastiche and again as a Solar Pons story, but I swear I’ve read (or heard or watched) it elsewhere as a story with another detective. I could swear it was Christie’s Poirot, but I guess it could just as easily be the dim memory of an Adrian Monk or Banacek episode.
If you have some idea where I might have encountered the story—maybe a radio adaptation?—I’d appreciate your telling me. It’s driving me crazy.