Think I've got to disagree. I had a particularly loverly example eariler: Was lying down, not moving, up against the edge of a non-enterable building(one of those large factory-looking things). Suddenly I hear a zombie aggro sound-effect from the direction of the building. Several moments(during which I still don't move because I don't know whats going on) later, two zombies come barreling around the corner after me. After I killed them I made sure to examine the building, and it is not enterable(at least not by players?). Some time later, I decide to just sprint across an open field through about six or zeds to a barn and just kill whatever chases me. Turns out only one of the zombies saw/heard me. Ran right in front of at least three of them, but they never aggroed. And of course I'm skipping all the times that zombies have aggro'd seemingly out of the blue from the far side of buildings or walls or managed to turn around and see you lying prone from over 100m away. Zombie aggro IS busted fairly well, and it's completely inconsistently busted at that.