Poor image, but here is map of one route Shane could have chosen to walk home. As you can see the distance is only 2.8miles.
In the report Brett is listed as showing up around 1:10am at the scene of the accident, talked with a deputy and then left. He was discovered next by LE on opposite side of levy looking for Shane. Safe to say that was around 1:25-1:30am?
If thats close, Shane had over 20-30mins of run/walking time if he left when he first saw LE showing up.
At 3mph he would have traveled over a mile in that time, almost half the 2.8mile distance to his house. By the time Brett got over the levy to look, shane potentially could have been way farther down road than Brett thought.
Additionally it seems possible (although improbable) that Shane could have walked home, packed, and left again after LE's 1st visit and before their 2nd.
I would focus search efforts in a roughly 1.8 mile radious of his home.