This was posted on Reddit, stating that Jim allowed another author to write Mac into another person's work. This means that Mac spans stories outside the DF universe. It makes a more powerful argument that he is similar to a "Corwin" like character that has an ability to pop into different literature. And that Jim may have 'borrowed' Mac from another person's work himself. Or that Mac appeared in a previous person's work prior to Jim writing him in DF, like say in Amber Chronicles.
Jim did allow an author to borrow Mac - Evil Ways by Justin Gustainis 2009
self.dresdenfiles
submitted 11 hours ago by Jai-Dal
Chapter 9 approx pages 160-165
Jim gets asked questions if he would like to play in other authors or IP’s playground or allow anyone to play in his. He has said he’s very possessive of his baby. Which is great! But I remember him mentioning that he allowed Justin to borrow a character. That’s why I bought the books by Justin to read. They are pretty good.
I think there are 3 in the Morris & Chastain investigations. (There may be a 4th book of 2 novellas combined) And then 2 books in the occult crimes unit series.
I’ve always wanted to hear what Jim thought or any comments he has on Mac in another book series.
I haven't read these books so I don't know much about it, but the reddit mentions Mac as a Watcher too.