I love how Flack is very much in his role as Homicide Detective, yet the part of his brain that he's kept under wraps for so long is itching to take over. The ambiguity in the act of taking the sheets of paper with his equations is perfect.

Behind the mirror, Danny’s eyes had to be crossing. The only one that knew where Flack had gone for college was Mac, and that was only because he had Flack’s file.

I desperately want to hear that conversation in Sullivan's. :)