Left 4 Dead 3 and Source 2 have both been spotted on Valve's internal project tracker, after members of the Dota 2 subreddit were given a tour of the company's Seattle offices.
The picture taken primarily shows updates to Dota 2, but nestled between are references to the Source 2 engine and a L4D3, and Reddit member Cyborgmatt uploaded a zoomed-in graphic to show the applicable entry up close.
The current Source engine made its public debut in 2004 with the launch of Half-Life 2 and Counter-Strike: Source, and has subsequently gone on to power Portal, Team Fortress 2, Left 4 Dead, and Dota 2.
Valve confirmed in November 2012 that it had been working on a new game engine for "quite a while" but suggested it was waiting on the right game before showing it off.
Left 4 Dead 3 was also reportedly spotted on a Valve project tracker in June.
Left 4 Dead 2 was originally released for PC and Xbox 360 in 2009, and has since been ported to Mac and Linux.