ValveTime says that the code had over 60 lines referencing the Source 2 Engine, but cited one particular line below.
”’Return an str with the current engine version.
If key doesn’t doesn’t exist, assume ‘Source’, otherwise invalid — assume next-gen ‘Source 2′.”’
In addition, a member on Facepunch found the following icons for future Source 2 tools hidden in Filmmaker’s files. These icons represent tools for scaling, painting, and terrain modelling, as indicated by their respective file names.
Valve has yet to comment on these discoveries, but Valve’s website says the Source Engine is “about to get even better.” In any case, there is never any doubt that Valve has something up its sleeve.