There was an English language article a few years ago making a very good case that the Surcouf was sunk by U-502 Von Rosensteil with his last torpedo on the patrol 3. Von Rosensteil said he saw a small low oil tanker like ship. He did not get credit for the tanker but the dates and the general location make for a very good possible. As I think on this the author said the location was 100km from where Surcouf should be. But like jtr said, who knows where the boat was. Von Rosensteil was very careful and did not boast. If he said he sank something, it sank.