Revealing mistake: It's a valiant effort, but the scenes of Sherlock & Irene in a London street are shot in New York. Not only is the architecture wrong but you can see a U.S. pedestrian crossing signal over Irene's shoulder. (00:16:15)
Revealing mistake: To demonstrate that "Clean This House" isn't a real company, Sherlock shows the man his phone displaying a 404 "file not found" message. Except the browser bar at the top simply shows "Clean This House", not a website domain. As written, he's just searched for the phrase, not tried to load a website.
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 ★