False Name
There are men in the world who are given papers, fully accredited and fully legal, that do not reflect their given name. M becomes J. And that is who he is. And he has to make his travel plans so as to avoid cities and countries where he might run into friends and family.
“I wanted to visit S, her husband, and her new baby, but I couldn't, “M told me once. She lived in Brussels and he was free from his last tactical operation and had some time to travel, but couldn't visit her because he was not M but was J. And national security could not condone this intentional stress on the fabric of reality. Better safe than sorry, as they say.