Sorry guest, this is a bug, whatever which way you look at it. Zombies can see you from too far away - fact. If I have to look down my scope to "see" a zombie, but yet this one has already seen me (with his, I'm guessing, deteriorated eye sight from being dead and all) and started running at me - there's a bug there.
Gotta love it when a 'guest' takes it upon themselves to moderate a forum. Good job.