Valve's announcement of a new "Mann vs Machine" game mode for Team Fortress 2 didn't just come out of the blue. It didn't even have its hat tipped over the weekend.
No, the studio that does crowd-teasing like no other has been building up to this moment for over two years.
The history of the update's promotion has been catalogued over on this Reddit post, which shows the build-up to Team Fortress 2's biggest update in years began all the way back in May 2010, with the release of the "Meet the Engineer" update. In the first 15 seconds there are several glimpses of robot heads, at the time seemingly harmless but now revealed to be the skulls of two of the game's new robot foes.
The paper-trail continues through the release of new maps (with hidden rooms containing clues to the impending robot army), updated game files containing further clues and even hidden links on the game's website.
It's exhaustive. If you needed further explanation as to why there's no impending Valve game with a "3" in the title, it's because they're all too busy being funny-buggers with the code to a game that's nearly five years old.