Let's just take a deep breath here. Lots of things are episodic - and if you don't have as many as three episodes, you're just a two-part TV special. So don't freak out when I say that some placeable AI hints in Alien Swarm refer to entities tagged with the "ep3" prefix, right after familiar hints for "hl1". Here's a pic:
It might not be Half-Life, but what if it is? Either ep3_blob is something we all know about that Valve call a 'blob' (like an advisor, maybe), or this is our first clue as to the new bad guys in Episode 3. Whatever it is, it spawns, spits, shakes, and it has a brain that needs to regenerate.
We're reasonably sure that Alien Swarm uses a version of source that split off from Portal 2's code around the time they made the E3 demo videos. The types of hint nodes available are all over the place - there's strider nodes, alien blood hints from Half-Life 1 (to be placed near a blood decal so that Scientists would know to remark on the smell), and all sorts of stuff that shouldn't be in an Alien Swarm game. It makes sense that something in development at Valve might have slipped through in that particular place.
You may now start drawing pictures of the blob and Gordon killing/saving it.