I was thinking about the timing as well. One possibility is that David was supposed to be unloading back in Sac City by, say, 1:00am or so. When he didn't show up by 2:00am or 3:00am, didn't Wiechman's call either dispatch, or the loading place to see if he'd ever loaded? She said the loading place was unaware of the situation.
Somehow Les, the man he works for, was made aware and she said Les went looking for him along alternate routes he may have taken. So I wonder if Les first drove the most likely route (Rt 20), didn't see him, and then decided to drive the other route or routes back and forth. That could potentially have taken an hour or so each way, so maybe Les was driving around from 5:00am to 9:00am, and then decided to head directly to their house?