I think he does have one; he has areas he researched and writes about (like tools for thinking) that I assume inform his reading. He talks on a podcast I listened to about how his notes drive his creative projects. I think he does talk a bit about how he does keep “hobby” notes, too, though.
That said, I think most of my note taking is me just following threads I am interested in. They often align with my work but not usually by design these days. Most of my work-related notes would just be meeting notes, unfortunately.