In a tutorial for Valve's new VR Tool titled Destinations, a developer may have shown off a bit more than he should have.
While this isn't the first time there has been a leak of Left 4 Dead 3 in the past few years, this is the first one that officially came from a current Valve employee. The employee in question, Tristan Reidford, has also played almost 22 hours of Left 4 Dead 2 in the past two weeks. Left for Dead 2 has also been receiving a lot of updates recently, leading more credence to the theory that Left 4 Dead 3 will be Valve's first game built from the ground up for Source 2.
We will update the story as we get more info.