In the movie Hercule's works, Poirot is finding who is Marrascaud the thief and killer. In one of the last scenes, when Marrascaud is taken by the police, Marrascaud turns and says to Poirot:
We will meet again
To what Poirot answers:
I will not turn back
So the question is: why does Marrascaud say that? I would suppose this is a reference to another movie or novel where Marrascaud is there, but I could not find such a plot.