The Niihau incident was probably related to espionage. They may have had some contacts on Ni'ihau, but this was discovered later on. The incident may have happened before or after December 7. The first mention of it that can be found on a newspaper dates back to the 1950's, at least as far as I was able to find out, so it's impossible to be sure exactly at what time the crash happened. Edit: The Ni'ihau incident also contains unlikely elements, such as a conflict occurring between the pilot and the locals over some secret document that the pilot was allegedly carrying. Would it make sense to take secret documents on a combat mission over enemy territory? But what if it was instructions to clandestine Japanese forces in the area? Take for instance the West Loch disaster. The remains of a Japanese midget sub were found in the vicinity afterwards. Maybe West Loch wasn't an accident.