It's just a man in a T-shirt of course, but there's a Half-Life 3 logo on the front of it and a Valve employee inside it.
Art director and executive producer at UberEnt (the team behind Super Monday Night Combat) posted the photo above on Twitter with the message "All I'm saying is I saw this at a local game developer event worn by a Valve employee," sensibly attaching a #ValveTrolling hashtag. He couldn't get any more info out of Valve's man but he confirms that "I did try to smother my face in his chest on the HL3 logo." This did not help.
A long time ago, when dinosaurs roamed the earth, Valve announced that they'd release three follow up episodes for Half-Life 2, allowing them to put out shorter stories at a faster clip. The episodes so far have taken longer and longer to develop, and Episode 2 ended up being longer than most modern shooter campaigns. Years on from Episode 2's release, it would make sense for the next game to ditch the episode format and become Half Life 3. Recently, the Cambridge Student asked Gabe Newell whether he considers releasing Half-Life 2 sequels in an episodic form a mistake. "Not yet," he said.
Valve have repeatedly said that they're still committed to the Half Life series. We'll surely see another one one day. WE JUST WANT TO KNOW HOW IT ENDS.