I don't find it hard to believe a rumor that Valve is working on a Source 2 engine. Now, if it was Source 3...or anything ending in 3, I'd call B.S. immediately.
He's found many more references to a "Source 2", but not just one vague line like our previous entry. We're only one file deep, and already it's clear that the references to "Source 2" are indeed referring to a next-gen engine that Valve is currently developing. I'm only going to show one line in this post, but there are about 60+ references here, and this one line is probably the most telling.