source: NeoGAF
Source2 and L4D3's existence were leaked a while back during The International 3
http://www.pcgamer.com/2013/08/06/left-4-dead-3-and-source-engine-2-reportedly-referenced-in-valve-changelog/changelogvalve/
source: NeoGAF
Source2 and L4D3's existence were leaked a while back during The International 3
http://www.pcgamer.com/2013/08/06/left-4-dead-3-and-source-engine-2-reportedly-referenced-in-valve-changelog/changelogvalve/