Valve has apparently pushed a file to Dota 2 Reborn called "hl3.txt" that includes a bunch of code that can be viewed here. Most interesting are the references to virtual reality, procedural generation, and what looks like co-op NPCs.
But knowing Valve, this is may just be a deliberate troll so it can have a good laugh at everyone online losing their minds yelling "Half-Life 3 confirmed!" or perhaps it was an honest mistake. Only time will tell.