Short answer: loads of new things have appeared in the DOTA 2 update and everyone's pointing at all the '2's.
The new files have appeared in the alpha release of DOTA 2′s Workshop Tools and will make it possible to "create, play, and share custom maps and game modes for Dota 2" say Valve. Currently the new stuff requires a fairly decent computer - a PC with a 64-bit version of Windows and a Direct3D 11 GPU - something people are saying indicates an incoming tech upgrade despite Valve saying the requirement will decrease with optimisation.
Most, if not all, of the Source 2 rumours have originated from this one Reddit post. There, RoyAwesome argues that the mod tools are actually a soft launch the for Source 2 engine because there are some '2's in some file names:
"Everything that would share a name with Source is named '...2'. That's 'engine2.dll', 'vconsole2.exe', 'vphysics2.dll'."
The Reddit post also argues that the way the new update handles .dll files and rendering has changed significantly from how the Source engine currently does things. Then there's this, showing the magical words 'source2' at the top of the file structure:
However, at the moment, that's about it - some circumstantial evidence and finger pointing. Speaking to Games Industry Rust and Garry's Mod creator Garry Newman isn't convinced, saying that the Source engine is already pretty modular. He explains that, "it's possible to upgrade certain elements like physics, sound, GUI, renderer, engine without having to upgrade the entire engine". As far as he's concerned this, "could be Source 2, or it could be that a bunch of components from Source 2 have been backported". Eurogamer's tech side also report that what people are seeing could simply be "tools that relate to the Source 2 engine to be used to mod Dota 2, rather than the engine proper".
That's the situation as it stands. With the exception of the input from Newman and Eurogamer every other story is built on that single Reddit post.
It's worth noting however that in his own Reddit AMA Gabe Newell was asked about the changes Source 2 would bring when it finally arrived and his answer was "increasing productivity of content creation" which does match up with this update's entire purpose.