When Jean-Louis Trintignant's character is being grilled, the policeman mentions that witnesses saw a man and a woman. Trintignant responds by whistling a bit of the theme from his previous movie with Claude Lelouche, the international sensation Un homme et une femme (1966).
The trivia item below may give away important plot points.
During the kidnapping of the bank employee's child, Simon tells the child he can refer to him as "Uncle Simca." This is reference to the fact that he has falsely convinced the family he is a representative from the now-defunct French car company SIMCA (which stood for "Société Industrielle de Mécanique et de Carrosserie Automobile"). When the term is spoken, however, it can also be heard as the Hebrew/Yiddish noun "simcha", meaning "festive occasion," which Simon provides for the happily oblivious child by spoiling him with presents and candy.