From other information, I think it was on Union Avenue (or 71 (kind of)) between 190th and 180th. This location also coincides with the LE release asking property owners to search their properties in NE Sac County.
The farm that he picked up from let them search all over outside but didn't want them in the facility, which is understandable because they are afraid of bringing disease in. I'm guessing, but I don't think anything happened at the pick up point.
They searched Brushy Creek (which is near where I grew up) for a reason. I suspect LE pointed them toward there and now they are searching west of Ft. Dodge and North of Rockwell City.
Personally, I think he will be found a few miles from the truck. All JMO.