According to a new video, Valve is indeed working on a new iteration of their Source engine, which supports references to "Source 2 engine" found in the Source Filmmaker app earlier this year .
Last week members of 4chan.org's video game community visited Valve headquarters to wish Gabe Newell a happy birthday. During a conversation, committed to video and embedded below, Newell is questioned on the existence of Source 2.
"We've been working on new engine stuff for a while," he reportedly said. "We've just been waiting for a game to roll it out with." When asked whether it was simply a makeover for the existing Source engine or a totally new engine, Newell reportedly responded "yeah" to the latter.