They don't appear linked to any established IP. They range from character concepts to ideas for spaceships, or at least very advanced fighters. What's Valve up to?
The short answer is we don't know and they're unlikely to spill anything yet. Of course videogame journalists taking various snaps of their in-house artwork had to have been 'sanctioned' by Valve as otherwise they'd have never made it out.
Below are some of the concept artworks.