The Gay Falcon

Ladies' man and amateur crime solver Gay Laurence (George Sanders), the "Gay Falcon", reluctantly agrees to give up both habits to mollify his fiancÃ©e, Elinor Benford (Nina Vale). He and his uncouth sidekick, Jonathan "Goldie" Locke (Allen Jenkins), become unenthusiastic stockbrokers. When Elinor asks him to attend a party given by Maxine Wood (Gladys Cooper) to mingle with potential clients, he refuses to go to that much trouble.
However, when Wood asks for his help via pretty assistant Helen Reed (Wendy Barrie), he cannot resist. It seems that Wood's soirÃ©es have been plagued by jewel thefts, and she is particularly worried about the diamond of her guest, Vera Gardner (Lucile Gleason).
At the party, Elinor becomes annoyed when she figures out why Gay changed his mind about attending, and retaliates by dancing with Manuel Retana (Turhan Bey). In frustration, she grabs the flower from Retana's lapel and flings it at Gay. He calmly picks it up and attaches it to his lapel. Vera Gardner then insists on dancing with Gay; she hands him her diamond secretly, much to his puzzlement, then leaves the room. Moments later, a shot rings out, and she is dead. The killer is seen by Goldie as he makes his getaway.
Police Detectives Bates (Edward Brophy) and Grimes (Eddie Dunn) take Goldie to the police station on suspicion of murder. Gay persuades Inspector Mike Waldeck (Arthur Shields) to release Goldie so he can flush out the real murderer. Then he and Helen go to see Maxine, leaving Goldie in the car. While they are gone, Goldie is abducted by Noel Weber (Damien O'Flynn), Gardner's killer. Weber orders Goldie to call Gay to offer to trade Goldie's life for the diamond. However, Weber is shot, and once again, Goldie is found by the police near a dead body.
By this point, Gay suspects Gardner arranged to have her diamond "stolen" so she could collect on the insurance. The flower was a signal, indicating to whom Gardner was to give the jewel. It should have been Retana. Gay and Helen break into his apartment, but have to hide when the owner enters. He realizes someone has been there and opens a secret compartment to check if it has been found. Relieved, he leaves the room. Gay sneaks in and takes a gun he finds in the compartment, fairly certain it was used to shoot Weber. The police confirm it is the murder weapon.
Meanwhile, Gay calls Elinor to warn her to stay away from the killer, but she believes he is lying out of jealousy and tells Retana so. Forewarned, Retana goes to Gay's apartment, ties up his servant Jerry (Willie Fung), and demands the diamond at gunpoint when Gay returns. Fortunately, he is frightened off when he mistakes Helen at the door for the police.
Now certain about his theory, Gay goes to see Maxine, taking Inspector Waldeck along. She tells them she has been receiving threats, so they stand guard in the living room while she sleeps. Retana enters through her bedroom window, but when he lunges at her, Gay and Waldeck charge in. They are puzzled when Retana collapses and dies. Then Gay finds a hypodermic needle on the floor. Gay stops Maxine from stepping on it and destroying the incriminating fingerprints. He reveals that she, her husband Weber, and Retana were responsible for the thefts. The Webers decided to betray Retana, but he found out. Gay realized she must be involved when Goldie was kidnapped; nobody else knew where Goldie was at the time.

Question: Who gets mistaken for the police at the door?
Answer: Helen