Actually in some cases, I think they call a GJ as a fact-finding thing - I seem to remember that they had at least one and I think several in the Ramsey case, and nothing ever came of that.
And personally, I suspect that the white truck seen on the access road may well have been the turf tech's, when he parked to close the gate. Looking at the photos from the LE site, that access road is somewhat higher than the actual paved road, but is well below the level of the school grounds. Depending on where they were located, someone looking that way might have seen only the upper part of the truck (and someone in or beside it), and not been able to see the wheels (or the trailer and equipment behind it).