According to some users over at Valvetime.net, if you look through the newly released Source Filmmaker, you can find files the have references to a "Source 2" and a new Half-Life game.
For those of you who don't know, Source Filmmaker is a tool that utilizes film-making tools within the source engine, allowing you to create films using the engine that supports games such as Half-Life 2, Team Fortress 2, Left 4 Dead, and more.
So far, each new engine made by Valve has been introduced with a Half-Life game, which is probably going to be the case with this engine. This is huge news if you've been waiting for the next installment in the Half-Life series, the last one having been released a little over five years ago(2007!).
Hopefully this is hinting a new Half-Life game in the near future for all of the fans who've been waiting!
Here's the file path on the hard drive: ".../SourceFilmmaker/game/sdktools/python/global/lib/site-packages/vproj/vproj.py"Valvetime article