Prop_hallucination?
So, I saw this page (https://developer.valvesoftware.com/wik ... lucination) on the wiki and immedeately though "COOL! I wanna try that out!!" and so I did.
So, I'm not sure how the hallucination prop is supposed to work, but it's not disappearing. At all. I expected the thing to stop drawing moments after it comes into the player's view screen. But instead, I can look right at it and, besides being creepy because I chose a model of Atlas and its not animating or casting any shadows, it doesn't do anything hallucinatory.
I just thought I'd ask and see if anyone has made it work, or if its simply reverted to a bad prop_dynamic in Portal 2 (which makes me a little sad).
According to the same VDC page, it has been removed from the Portal 2 FGD. I'm not quite sure on the specifics of that but I think the gist of it is that Hammer can't "find" the entity, or make it work, or something.
Anyway, I think that could be your problem. Of course, that can be worked around with custom FGDs. Or you could use a system of trigger_looks and so on.
you can check to see if the engine still supports it, fgds just are a list of what is to show up in hammer
The engine seems to support it, did you copy the FGD code from Alien Swarm? There are some properties that control the fade that might have strange defaults.
An easy fix to the FGD instead of created a custom one, just add the Alien Swarm FGD to your Hammer config, and put it under Portal 2's. Then use as normal. If the engine support it, it will work. If not, you'll know. 
http://www.tophattwaffle.com/ - My site for Source Engine Level Design
http://therewillbescience.com/ - Portal 2, Lab Rat
http://www.moddb.com/mods/portal-2-lab-rat - Portal 2, Lab Rat (MODDB)
Ooh, good idea. I didn't even know you could add extra QCs, I had just been adding #include directives in base.fgd.
Yeah, tools>options>add
http://www.tophattwaffle.com/ - My site for Source Engine Level Design
http://therewillbescience.com/ - Portal 2, Lab Rat
http://www.moddb.com/mods/portal-2-lab-rat - Portal 2, Lab Rat (MODDB)