Factual error: Many shots, especially the early ones, include the iconic Renault 16 in the background. This car was first produced in c1965, three years after the events happened. Probably unavoidable, as the car was a huge success.
Factual error: When the Jackal is driving into France in his 1963-correct Alfa Romeo Giulietta, he passes a red Alfa Duetto. Those were not made until 1966. Likewise the Renault 12 or the electric locomotives.
Factual error: When The Jackal arrives in the Paris train station, a modern luggage cart is seen, similar to those commonly seen in airports today. They were not in use in 1963.
Factual error: Charles de Gaulle's Citroen DS19, a vehicle that appears repeatedly, is an anachronism: it's a 1965 model used in a movie set in 1963. Although the design differences are few, they are noticeable from the outside. It's difficult to imagine why the production team used an incorrect car that got so much screen time (as opposed to the aforementioned 1966 Alfa and several other background vehicles).
Answer: He would have just slipped away into the crowd, probably with some minor alteration to his appearance. He was a master of disguise and no one knew his real identity or what he looked like.
raywest ★