Valve has a long history of leaks. Ranging from the theft of its Half-Life 2 codebase to the premature announcement of some of its latest titles, Valve can never seem to catch a break.
Today, it seems that word of Left 4 Dead 3 and the Source Engine 2 have been leaked during a tour of Valve HQ, wherein a developer's monitor was apparently—or accidentally—left open on a list of Valve projects. One of the tour members claims to have taken a photograph of the monitor and naturally, he posted it on reddit (hi-res image).
At this point, it's unverifiable as to whether the photograph is legitimate. Judging from the names on the list, it may well be legit, as several of the names are of known Valve developers, such as Ken Birdwell (via GameFront) and Jeff Sorensen.
Once again, we would like to remind readers that this being the internet, it's easy to doctor pages like this by going down a list of names of developers from MobyGames and LinkedIn and adding the necessary text to make it look legitimate, so we suggest taking it all with a grain of salt.
We await comment from Valve.