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...
Some interesting points, sounds plausible that Valve would do more work with Portal in Source 2 concidering they already used Portal for their VR experiments, does seem a little odd that they would mention it on a tour though without officially announcing it.
I wonder if the 3rd VR project they...