Weird - just got this same question over on Nexus...

I'll just copy and paste my answer from Nexus here.....
I'm in the process of doing several video tutorials on this and several other nifty Prefab tricks I've discovered to enhance POI game-play (hidden noise traps, sleeper spawning under water, hidden mines, etc.). But the short version is that this is supported by columns of terrain blocks that have had their 'density' manipulated in the Prefab Editor (using the down arrow key) to the point where they are no longer visible. In this state, they still have SI, and neither the zombies nor the players can directly interact with them. Zombies will not agro against them as they don't even know they are there. There is a particular property with terrain blocks that makes this possible - which I will explain in detail in the tutorial.