I was looking at my original post about Mad Men Episode 6.04: To Have and to Hold, and found this:
There’s also something in this episode about secrets getting out, which does not bode well for Don and Sylvia. Joan’s secret liaison with Herb Rennet is known to Harry. Ironically, Ken, who is so angry about “Project K” being kept a secret from him, is the one who leaked word about how Joan got her partnership. (Ken was at the dinner with Herb and Pete where the proposition first came up, so Ken was the only non-partner to know.) Joan’s partnership, Lane’s suicide, Project K’s meeting, Peggy taking Stan’s intel, Scarlett sneaking out…eventually everyone’s sub rosa activity is out on the table, as bold as Mel and Arlene.
I could never have guessed it would be Sally, though.