At this point you have to wonder whether Valve are just messing with us. Strings referencing "HL3" and "L4D3" have been found in an early version of the SteamVR Performance Test, along with a high quality model of Dog from Half-Life 2.
We have a cool list of upcoming PC games, and there isn't a single bloody Valve game in it. It's a travesty.
The SteamVR Performance Test is a simple tool that tells you whether your PC is capable of running VR at the required fidelity. Valve released a version of this software a little too early this morning, and it was removed soon after, but not before people went digging through the files like rabid masochists.
Among other stuff, a high quality 3D model of Dog from Half-Life 2 was uncovered. It's far more complex than the model used in the existing Half-Life games, with wires, gears and new armour plating adding a layer of detail the character lacked before.
The content has since been removed from the current version of the SteamVR Performance Test, and as sad as it makes me, this stuff is probably all really old, perhaps when Valve were experimenting with VR intially. Maybe back when Valve wanted to release great games instead of just constantly teasing us and making us sad all the time.
Basically: don't get too excited, we've been here before.