It's difficult to get too excited about new AAA games from Valve these days, with the most recent disappointment being the expected "full AAA" Portal VR game turning out to be nothing more than a bit of a tech demo.
Still, fans live in hope, especially about a new Half-Life game, and now it seems that something else is going on within the depths of Valve, allegedly code-named Citadel. The Citadel was a key feature in the Half-Life 2 storyline and it's subsequent DLC episodes, of course.
The code-name was found buried in a low-level engine update to Dota 2, which was discovered by the community and reported initially by Valve News Network. They have produced a video, below, explaining what we know so far.
It could still be a reference to the Half-Life VR project, but according to VNN, it seems more likely to be a brand new project built in the Source 2 engine. The code apparently contains a lot of references to stealth mechanics, AI and mini-map data.
While the name may just be a word used to describe the updates made to the engine, the phrase "Citadel" is certainly an interesting one. This by no means confirms anything, though, and these updates could be related to any upcoming Valve game using the Source 2 engine.