Police were alerted when conducting an immigration-related investigation into the Chinese wife of a JMSDF officer. During the search they came across the data, which included the radar and transmission frequencies of the Aegis system. The officer wasn't authorized to be in possession of the data so the investigation was begun.
He apparently came into possession of the data while swapping pornography with another JMSDF officer, according to a Japanese news source, the Yomiuri Shimbun newspaper.
Aegis is an American defense initiative.
Japan would like access to technical details of the F22A Raptor, one of the most technically advanced jets in the U.S. fleet. The U.S. has not accepted the request yet.
I guess this means you can't always trust a wife, or who you can swap pornography with.