I forgot to point out

You dismiss the whole point of that scene. The zombie was a relevant plot device, showing that he is not able to kill either walkers or Maggie. That instead she even saved his life when he treathened hers before.

It's necessary to show the occasional reminder that, out there in the open, it's still a barren wasteland of wondering dead. If you never saw walkers out in the open (even if it's just a loner)...you'd start to wonder why they even bother living in gated, cordoned-off communities anymore. It's important to show the world is still overrun by the dead. Plus, that scene with Maggie and the Hilltop boss was a plot-point. In just that little scene alone....we learn that he's never killed a walker....we see him contemplating killing Maggie (which shows us he realizes his leadership is slipping away).......the group of Hilltop folks walking by sees Maggie's prowess and their leader's weakness......we see Maggie trying to extend an olive branch to him, etc, etc...

In other words, a lot of decent stuff transpires in that little scene. A lot more than just a random walker attack. The walker(s) attacking was just a means of moving the plot-point along.

As far as suspending disbelief when a walker comes out of nowhere..............hey, we're talking about a show centered around the dead walking. If you can suspend disbelief for THAT, you should be able to forgive the occasional walker appearing from out of nowhere.