Valve's movie-making tool Source Filmmaker contains references to Source 2, seemingly a next-gen engine in development at the company.
Over 60 references to Source 2 were uncovered by Valvetime in the Source Filmmaker script files. One reads: "Return an str with the current engine version. If key doesn't exist, assume 'Source', otherwise invalid - assume next-gen 'Source 2'."
There are references to Source, Source 2, and Hybrid, which Valvetime posits is the Source 2 codebase. It believes Source 2 and Filmmaker were in development at the same time, which is why the latter contains so many references to the former.
Valve itself appears to be fanning the flames, with the description of the engine on the company's website reading: "Source is considered the most flexible, comprehensive, and powerful game development environment out there. And it's about to get even better."
And, with recent history in mind, it's hard to see this as anything other than a deliberate move from Valve to let the world know what it's working on. It's more secretive than most, of course, but so far this year the company has been opening up a little, albeit unofficially. There was the "leaked" Valve employee handbook, the Michael Abrash "wearable computing" blog post, and Gabe Newell's two-hour interview with a little-known podcast team.
Newell and company surely knew that, following the release of Source Filmmaker, the first order of business for its community would be mining the code for hints at what Valve is working on. It'd certainly be a very Valve way of announcing something - it famously revealed Portal 2 through updates to the original game, with a subsequent ARG, The Potato Sack, resulting in the game being unlocked on Steam hours before its intended launch.