Clearly the only way we’re going to get any information on the third installment for Left 4 Dead and Half-Life are if we ourselves invade their offices. In this case it were members of the Dota 2 subReddit team who were invited by Valve personally for a tour of the office.
In doing so the members took plenty of snaps and back at home one of the pictures revealed considerable evidence pointing towards Left 4 Dead 3. I take it another invade must take place to search for Half-Life 3.
The picture was of a computer screen displaying Valve change logs. When zoomed in, we can clearly see it mention “L4D3” and “Source2”.
The Source 2 engine was already semi-confirmed by Valve boss Gabe Newell back in November. He stated that the studio was in fact “working on Valve’s new engine stuff for a while”; after which he added that “we’re probably just waiting for a game to roll it out with”.
As things stand, Left 4 Dead 3 might be the debut title for their Source 2 engine. However, Valve still needs to do an official announcements and those do come so rarely.