So it seems there are numerous hints and references to Valve's next-gen engine called "Source 2" in some files of Source Filmmaker. Apparently SFM was in development at the same time Source 2 was, so there are some lines of code and stuff having to do with Source 2.
There are also unused icons and such to be used for Source 2 hidden in the SFM files as well.
You can read about the whole thing
here.