Ce n'est rien. Je me suis trompe

An unknown person speaks this quote; found in the conclusion of Chapter 4, Part I. Translated from French, the quote means, "It is nothing. I am mistaken." Poirot hears someone say this quote from inside Ratchett's room the night of the murder. Poirot hears Ratchett's bell go off, the conductor responds, knocks on his door and the person inside tells him that, "It is nothing..." Hector McQueen, Ratchett's secretary, repeatedly tells Poirot that Ratchett spoke no French. Thus, it is clear that whoever was speaking to the conductor, in French, through Ratchett's door was not Ratchett, but was most likely the murderer. McQueen's insistence that Ratchett knew no French implicates him in the murder, McQueen not only knew that someone had spoken in French to the conductor from Ratchett's room on the night of the murder, but he knew it was an important element in the investigation. This is another tactical element the Armstrong family uses to obscure the case, apparently another unexplainable element in the case pointing to an outside intruder.