Mysterious Valve concept art has emerged, after a fan and PC Gamer took snaps of said artwork in the developer's office.
There are 28 pieces in total, some of which you can see on this page, and precisely none of them look like anything we've seen from the Half-Life developer before.
PC Gamer reports that speculation over the art linked it to Doug Church (now a Valve employee) and the abandoned EA / Spielberg project LMNO - a few glimpses of which showed similar alien faces.
Other suggestions include Dota2 and, of course, an entirely new IP altogether (Left 4 Dead in Space??)
Back in September last year, Valve teased that three "big surprises" would be revealed in the next 12 months. One of those appears to be the second DoTA game, but the developer has remained extremely coy over any talk of a new Half-Life.
As our sister publication points out though, Valve doesn't 'accidentally' leak concept art, so either this is a big tease or a prank altogether.