Files found within Valve's Source Filmmaker program reference a next-gen engine from the company, referred to as Source 2.
Numerous lines of code reference Source 2, as uncovered by Valvetime.net, suggesting the new engine is in development and primed for inclusion within Filmmaker at a later date.
"Return an str with the current engine version. If key doesn't doesn't exist, assume 'Source', otherwise invalid -- assume next-gen 'Source 2'," one snippet of the Matrix reads.
The original Source engine has proved an adaptable and long-lived beast. It originally debuted with Counter-Strike: Source and Half-Life 2 in 2004 (yes, it really was that long ago).
More recent Valve games have included newer iterations of the engine, dotted with various improvements such as better facial animation and support for different platforms.
The above code suggests that Valve are working on a new and entirely separate Source build for use in future games. Maybe even Half-Life 3.
Valve officially launched Source Filmmaker last month on Steam. The free software allows users to create movies within the Source engine, using assets from games such as Team Fortress 2.
"We don't like to brag, but Source is considered the most flexible, comprehensive, and powerful game development environment out there," Valve states on its official site. "And it's about to get even better."