At least in the mod, non agro'd zombies had a very slight "Pull" to survivors even if they didn't see them,
For example, if you hid for an extended period of time, the zombies would slowly "Gravitate" to your position. They wouldn't RUSH you, but they'd ever so slightly move towards you as they wander around.
This behavior MIGHT be in the standalone, as well. Not really a bug, but this behavior should be changed once zombie AI is fixed up.