What Valve is working on at any one time is often a point of contention for fans. There's the rumoured Half Life 3, another Portal, a Left 4 Dead sequel, it could all be being developed or none of it, thanks to Valve's disinterest in far flung teases and mid-development announcements, something that's far from common in the contemporary gaming industry.
However, something that Valve has been confirmed to be working on for some time, is a sequel to its Source engine, Source 2 and it looks like it might have soft-launched it with no fanfare whatsoever, porting DotA 2 over to it in the process.
Released alongside the alpha of DotA 2's Workshop tools, was a big update for DotA's Hammer map editor, but the changes are far and beyond changing its version number, it appears to be running an entirely new engine.
As the source reverse engineering groups on Reddit have pointed out, key files like 'engine2.dll', 'vconsole2.exe' and 'vphysics2.dll' all point towards a new version of the engine, alongside the editing tools themselves.
There's also several big changes made that upgrade the abilities of the engine far beyond what it was capable of doing just a few days ago, which would certainly point to a new version of Source.
System requirements have also taken a jump, though some have suggested that that could be mostly to do with a lack of optimisation as of yet. Everything looks and plays the same, but for some, frame rates have really dropped.