Well this is certainly interesting. Tourists at Valve's Bellevue-based headquarters may have inadvertently leaked the existence of Left 4 Dead 3, a sequel that is not entirely unexpected, but still unannounced.
Check out the following photo, shot by members of the Dota 2 Reddit community, who were invited to visit Valve while in town for the Dota 2 International this week.
And here's a closer look:
Perhaps the next game on Valve's slate is the third entry in their popular co-op zombie-shooting series? Sadly, there's no mention of Half-Life 3 in there, although I'm sure you can piece together an ARG if you try hard enough.