When sneaking along in the tunnels, under urban areas, the zombies alert as if you are walking right by them on the surface. There seems to be no vertical line-of-sight separation between the underground areas and the surface terrain. I even had one zombie follow me to the rope ladder and dropped down the hole into the tunnel with me.

Find a tunnel under an urban area...tip-toe through that tunnel and zombies on the surface will AGRO on you as if you are walking right by them on the surface.