This time, it comes by way of a Dota 2 update. In a new update, a file called hl3.txt has been found, and it makes reference to a lot assets in the game. There is mention of VR, procedural generation and co-op.
Honestly, I don't quite know what to make of this. To see this turn up in a Dota 2 update…I feel this must be deliberate. It's just a case of are Valve brash enough to mess with their fans like that? And honestly… I wouldn't put it past them having a bit of a joke.