Character mistake: Clyde is a tortoise, but Joan, after finding Sherlock, put him in her bed, brings him down and calls him his "pet turtle." (00:14:08)
Visible crew/equipment: Sherlock and Joan are sitting on a street restaurant. The restaurant glass door opens in the background and you can see a light box reflected on it.
Chosen answer: M broke into the brownstone and left a note. Holmes deduced he was staying at a high-class hotel from the soap scent on the note, and gave a picture from a hidden camera to his "irregulars" on the street to stake out hotels. One of them saw him, so Holmes followed him to his next intended victim and knocked him unconscious with a baton.
Sierra1 ★