A group of Dota 2 players invited to tour Valve's offices have apparently spotted references to Left 4 Dead 3 and Source 2.
A zoomed in version, via NeoGAF.
While Left 4 Dead 3 has not been officially announced, Source 2 has been rumoured to be in development for at least two years. Gabe Newell effectively confirmed that the engine is in development during a fan tour in November last year.
Tellingly, the changelog contains no reference to Half-Life 3. Valve recently poured cold water on rumours that the company had determined a release date for the long-awaited installment.
Left 4 Dead 2 released in 2009. At the time CVG wrote that "L4D2's core philosophy is to drag you, kicking and screaming, out of your comfort zone."