Half-Life 2 Behind object outline

So yeah I have been wondering for while now about how to create that epic background outline like
when you play L4D and your team mates are behind wall or something alike they lightout blue or other border around their character
or in TF2 you get that same effect for MVM Robot holding bomb, this effect is used in valve games alot, I am wondering,
how its achived, I had tryed placing 2 cams and 2 blocks,
one on special layer and one of the cams in child of main cam but only renders special layer and dosent clear
however main cam ignores special layer and does work like normal maincam... but that didnt do the trick

 

If you want to save yourself some time, there's one on the asset store for $5.

http://forum.unity3d.com/threads/144825-Wall-Vision-Outline-Image-Effect

 

This one is nothing like what Valve has. This is just a fresnel, the one in L4D is a true outline on things. If you clip through the model it will even outline the parts you're clipping.
I don't know how it's done but maybe you can render all objects to be outlined in a pass where black = no outline and then various colors for various things to outline. After that the post shader takes that pass and extrudes an outline along the flat colored objects. I'd love to try to implement that but my shader coding is very poor.

 

Take a look at this from the wiki. This could give you a good base for what you want.