An account I read or heard (sorry - can't recall where) gave me the impression that she and the kids were along a path. It's been called a 'trail' by some but what I picture isn't a groomed trail like you'd see in a park, but rather a path that would simply be the way of least resistance that would become beaten down because animals and people take it. Meaning... no felled trees in the way, leading to a pond or other water source, etc.
And what I heard (yesterday's presser, maybe?) indicated that the kids were doing as kids do when out exploring and walking with an adult - running ahead, running behind, etc. - not walking along with grandma holding her hand.
If they were just out doing their thing and everyone was feeling comfortable and at ease in a familiar environment, not expecting anything like a disappearance to occur, Grandma would have an awareness of he was running ahead or behind, but wouldn't necessarily have her ears peeled to listen for crunching leaves.
My guess is that the sister needed a shoe tied or tripped and fell and needed comforted - something as benign as that - and that Noah continued on down the path, darting in and out of trees, and she just planned to catch on up with him as soon as the sister's immediate need was taken care of. If their property is like the terrain I've experienced in that area, it's certainly not flat and if he ran ahead just a bit he could be up and over a little hill and out of sight, and under normal circumstances that wouldn't be cause for worry. You just catch up with them or they run back towards you.
If you've listened to the Serial podcast, you'll remember how in the first episode the host leads you to understand that when you're just living out a normal day, little details don't necessarily stick with you. In hindsight people assume when you can't remember something that it's a sign that you're hiding something, but in reality, if someone asked you where you were at 2:54 yesterday, unless you were known to be in a class or at your desk or something, you probably wouldn't be able to remember with specificity who you were talking to and what you were talking about, etc. When something goes wrong we expect every detail, down to the specific minute things happened, to be there with clarity. But life really just doesn't usually happen that way.
