Peter König, a Valve artist, has been responsible for producing concept art, models, and textures for many major Valve games including Left 4 Dead 2 and DOTA2. His personal website has a number of L4D2 concepts and a space shuttle with "SOB 49" stenciled on the hull.
According to HalfLife2.net, which spotted the news, last year's DOTA2 leak contained previously unreleased information about an IP in development at Valve codenamed "SOB", which suggested that the game was set in space. The image of the spaceship can be found on his current website under the Hard Surface / 3D Concepts section.
This leads us to speculate that the artwork may well be for Valve's IP, which remains in limbo as of this time. It's no secret that Valve throws many game concepts against the wall and goes with whatever sticks—so this one might've ended up on the floor.
In an e-mail to HalfLife2.net, Peter König said that some of the artwork on the website is indeed his, but not all. He added that although he is unable to comment on the nature of the artwork, the fact that much of it is on his website means it can't be too sensitive—which I suppose is his way of saying that whatever he was working on was scrapped and isn't a big deal anymore.