The Last of Mrs. Cheyney (1937)

Mrs. Fay Cheyney, a rich American widow, insinuates herself into London society. Two men in particular -- middle-aged Lord Kelton and Lord Arthur Dilling, a young playboy -- pursue her. All are present at a large weekend house party, and though both men press their suit, Fay seems to favor Kelton. Then Dilling sees Fay's butler lurking in the gardens, recognizes him as a jewel thief who was apprehended in Monte Carlo, and realizes that Fay is probably after the hostess's pearls. Fay does get hold of the pearls -- but before she can pass them to her accomplice, Dilling gets hold of her.