latest experimental build (3-22-2014) across 2-3 servers over the course of the evening. I've noticed individual and groups of zombies standing around attacking something invisible. They do not react to me or stop attacking to come after me, but if you approach them, at around 5-8ft distance you'll start getting hit by something. Zombies are still focused on attacking the invisible object and do not turn or react, but damage is being taken. I saw numerous zombies, both individual and large packs doing this.