This has only happened twice, but I've experienced situations in which I am attacked by zombies that are not visible in either 3rd or 1st person view.

The attack is obvious, as the player takes damage, blood is shown, and aggravated zombie noises can be heard.

I've only encountered this near Nadezhdino near the Northmost building, directly east and slightly south of the deer stand.

The location can be found at the center of the map shown in this link: http://dayzdb.com/map#7.058.104