Valve's Tristan Reidford has lightly teased Left 4 Dead 3 in a tutorial for the Destinations Workshop Tools. The eighth image in the tutorial, titled "08_directory_fun.png," showed a conveniently placed folder called "left4dead3." From the pessimist's perspective, this was just a mistake, but it's too convenient that the folder viewport was scrolled exactly so that left4dead3 was shown first. According to Tristan's Valve Developer Wiki contribution history, the image was first uploaded on June 28, and then nearly 24 hours later, after the community had noticed the tease, it was replaced by an obviously Photoshop'd version to remove the Left 4 Dead 3 reference. Tristan Reidford and other Valve developers have been "playing" a lot of Left 4 Dead 2 recently. Currently, Tristan's Steam profile says he's played about 348 hours of Left 4 Dead 2 in the past two weeks. Also, Valve have released at least nine updates for the franchise in this year alone.
I won't be surprised if it's another "ha ha , you're an idiot" joke considering the other crap last few years .
It's an image called "directory fun" which just conveniently shows a folder called "left4dead3", plus this guy has been playing hundreds of hours of a hidden VR game on Steam when all the rumours so far have been that L4D3 will be the title to not only showcase Source 2, but the HTC Vive as well... I really think this could be it.
Looks more like it's an internal package to test various VR games and such. https://steamdb.info/sub/83209/history/
agreed something is off about what he was playing. the plot thickens... meh when valve do make new games, the poking media of gossip gets old real quick. am just content they are active.