As soon as they left the forest, there was a noticeable amount of zombies around the area already. Unlike humans, who were smart enough to not make a lot of noise at night, the creatures within the forest were not as cautious, their screeches and foraging for food at night continued to attract the attention of nearby undead.
With that being said, the constant sounds should have become a beacon, drawing a massive amount of them in. And yet, the amount of zombies wasn't comparable to the common sense of how many should actually be there. The reason didn't escape Noah, for zombies, their only goal was to eat and feed on humans, at least that was the common consequence he had gathered over the last few days.
