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.
- By going the other way? Before Pearl Harbor, they could cross the Pacific, then the Indian Ocean and get to the Persian Gulf. Then up thru modern Iraq and then cross all of Turkey. It would be quite a long trip, but doable.
- Mid-1941? On a ship flying the US flag. The US was not at war until December of that year. Conversely, on a ship flying the Turkish flag, as Turkey never entered WWII.