I assume it comes in the Dengar Villain expansion pack on a separate piece of paper. Same with the others you mention like the RGC. Since they come from expansion packs they aren't in the campaign books.
The Agenda Card tells you which expansion pack the side-mission is in. In this case "Dengar, Ruthless Killer Pack."
Yep exactly, the cards tell you check the pack they came in. They all come with a sheet that details their Campaign and Skirmish missions.