Left 4 Dead 3 in development?

During a recent tour of Valve's offices, a group of {Dota 2} players seem to have unearthed evidence that {Left 4 Dead 3} and Source 2 are actively in development.

One member of the group snapped a picture (which you can see below, the zoomed in version of the image that came via Neogaf), and both Source 2 (the next iteration of the engine used to power Valve's games) and {Left 4 Dead 3} were mentioned on the internal changelog.

Source 2 was confirmed by Gabe Newell last year, so it's not much of a surprise that Valve are working hard on the engine, but {Left 4 Dead 3} is a different kettle of fish altogether. Sadly, there was no mention of {Half-Life 3} on the changelog, but that hasn't stopped the speculation from rumbling on.