Joel Cairo is a refugee running from the Nazis.He has some kind of Eastern European accent. It's the 1940s. He has several false passports. He is a homosexual. You do the math.
How the heck are the villains going to get back to Istanbul?In mid 1941, the Mediterranean Sea is totally blocked by war, and even the long way around Africa is fraught with u-boat peril. When the book was written (in the late 1920s), this wasn't a problem. In 1941? It might be harder to get into Istanbul than Berlin.