In the latest episode of my history podcast about Pearl Harbor, I focused on the Ni'ihau Incident.
I spent a lot of time giving thought to what Shigenori Nishikaichi felt that his end game might be in the situation. Surely, after a submarine never arrived to pick him up, he must have known that he wasn't getting off of the island alive, but for some reason, he kept searching for Howard Kaleohano, the man who had pulled him from his plane and taken his "papers."
Nishikaichi even went so far as to essentially hold the entire village of Pu'uwai hostage in an effort to get them back. What could have possibly been contained in the papers that would lead him to resorting to these lengths to retrieve them?
Most accounts of this (in both English and Japanese) are pretty vague about this, just referring to "papers" or "maps and papers."
The most detailed (but still partial) descriptions I could find were in Nobuhiro Adachi's Hawai Nikkeijinshi and Mori Shiro's Unmei no Yoake. These describe the papers as including maps of Pearl Harbor and Hickam Field, random number tables for sending encoded telegrams, his orders, and "tactical abbreviations" (I'm not sure what this refers to). Mori also states that "there are various theories concerning what these 'papers' were", though, so even this account is obviously incomplete.
I don't think what was in the papers was as important as the fact that he had apparently been told that he wasn't to allow them to fall into the hands of the Americans. That seems like all the motivation that would be necessary for Nishikaichi to go to extremes to get them back.