Oh those wonderful people in Valve's forums have done it again. Upon scouring the Alien Swarm SDK (Source Development Kit) an oddly named group of attributes in a single file were found. The attributes were amidst a clump of Half-Life related attributes, tagged with the label "Ep3". The file doesn't seem to be in use in Alien Swarm, but who cares! This confirms that at some point in time Valve was working on Episode 3.

Take a look at a screen capture one of the forum warriors produced (click to enlarge):

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Right, okay, it's not much to look at and the attribute names aren't exactly forthcoming with any Episode 3 details. Still, it's something isn't it? We may never know what this 'Blob' is, that is referenced in the labels, but you can bet that at some point in time it was in Half-Life 2: Episode 3.

Now is the part where we start to wonder: Did Valve do this on purpose? Is it possible that Valve, tricky, tricky Valve, snuck in this file just so we'd be here today pondering its veracity? I wouldn't put that past them, what with all of their past hi-jinx and red herrings. In fact, perhaps this isn't the end of the rabbit hole. Perhaps we need to open the file in Google Maps, travel to the location that is marked, then open the box which will get us a new Half-Life 2: Episode 3 related hat in  Team Fortress 2.

I'm choosing a different method of dealing with this news, the method of muttering curse words under my breath while I play through Episode 2 one more time. You win this round, Valve. You win this round... again.

Source: Valve Forums