Fans from the Reddit DOTA 2 community went on a tour of Valve recently and some secrets may have been spilled among the many photos that were taken. A picture of an employee's computer screen was captured, and upon closer inspection you can see that Left 4 Dead 3 was mentioned. That, and it's going to be running on the Source 2 engine.
Source 2 is no surprise as Gabe Newell has talked about this before, but this is the first time that Left 4 Dead 3 has ever been brought up. I wonder if fans will get pissed off again that a sequel is happening again.
Anyway, we would reach out to Valve's Doug Lombardi for an official statement on the matter but we'd have better luck yelling at the moon for a reply.