I've heard several versions of the story about how the film people contacted Chandler, who replied that he didn't know who murdered the chauffeur.
In some of the stories, Howard Hawkes cabled Chandler. In others, it was the two screenwriters, William Faulkner and Leigh Brackett, who phoned Chandler. The author replied that the answer was "right there in the book", but later called back to admit the it wasn't, and he didn't know what it was.
By the way, I once read one of Raymond Chandler's very early short stories, called (if I remember right) Killer in the Rain. He wrote it before he was an established author - probably when he was still working in the oil industry.
The story was very much a dry-run for The Big Sleep. It had the same hard-boiled private eye (but he wasn't called Philip Marlowe), the same General Sternwood character in his orchid hot-house, the same dysfunctional daughters - and the same scene where the narrator is got out of bed in the middle of the night and invited to drive to the ocean-front, where a car had gone over the pier.
Sure enough, it was the same chauffeur being murdered. And, sure enough, the story finished without the identity of the murderer being revealed.
You'd think he'd learn from his mistakes.