Elocsol,
1. Kevin didn't really go out of his way to talk to a cop that night. It was by chance he/gf saw a St. Joseph's car sitting at Cotton's dentist office by the Tom Thumb. At that point they were more curious/nosy, but did mention they saw bikes on the road by the Rassier farm (which, again, the bikes were in a ditch across the road from the Rassier farm. Imo that proximity is key.
OK, maybe not so much out of his way - but the point is, he did pull into a driveway to tell a cop what he had seen.
2. According to Kevin's story, his brother told him the next day that the FBI was making plaster cases of tracks found in the driveway. That should have been more than enough information for one of those 4 people who knew Kevin/gf drove down that driveway. A white van was publicly sought and any information relating to the scene that night...
A van was sought, but it was a light brown or light tan van, not white, and that van was seen several times, months earlier, over by Kraemer Lake. It was not reported to have been seen in the Wetterling neighborhood. Even so, if LE was looking for a van, it means they weren't looking for his girlfriend's silver colored Pontiac.....