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.