LambdaGeneration has word on Valve's software engineering efforts based on a video of a visit to Valve by some members of 4chan's /v/ (thanks Joao). This seems to offer the first concrete word on hints that they are working on a new game engine. Here's their transcript of the pertinent passage:
All trademarks are properties of their respective owners.
News CGI copyright © 1999-2015 James "furn" Furness &
All rights reserved.
Chatbear v1.4.0/blue++: Page generated 28 August 2015, 09:30.