On the sources. Fightful actually was one of them. There is also an insider on SquaredCircle Reddit who has been very reliable who confirmed both that people were told ahead of time and Rollins/Punk.
Below is from Fightful:
Full details at Fightful Select
-Fightful was told on November 19 that CM Punk and Triple H had a very productive, hour-long conversation that mended fences between the two.
-Triple H confirmed Fightful's previous reports that talks had not happened between the two sides until rumors heavily died down, and it actually didn't look like a deal was happening. WWE sources say the deal came together quickly and a contract wasn’t signed as of Saturday AM
-Punk had told select people close to him that the return was happening.
-Minutes before Punk appeared, a section of backstage was cleared out, but Punk was seen in plain sight. We're told that Triple H took over producing the show himself at that point, called for the copyright graphic, and then also personally called for CM Punk's music to hit
-Producers, referees, staff, talent, and even head writers on the creative team were not told.
-Triple H, Bruce Prichard and Michael Hayes needed to discuss a change to the main event, but we weren't told specifically if that was Punk related.
-Main event talent being upset, there are numerous talent on the roster in general upset, just as there are some excited, curious and about every emotion. Talent were pulled aside before the WarGames match and told that CM Punk would be returning at the conclusion of the show.