It looks like Valve may have accidentally outed Left 4 Dead 3 prematurely. During a tour of the company offices for Dota 2 players, one crafty individual snapped a photo and shared it with the world on Reddit. Check out the pic below.
If you look really, really closely you'll see both Left 4 Dead 3 and Source 2 mentioned.
Of course, this doesn't officially confirm anything, as Valve has yet to make an announcement regarding either. As such, sit tight and keep checking back at GameRevolution for more info.