Good point about the scent stopping, and that may very well be why they think a car is involved, but here's my thought.
The police returned a few days later to search more, and from what I read here, and what I saw on tv, they were searching an area up the road from the cart road. From the way it looked to me, there were guard rails on both sides of the road where they were searching. Not enough room on either side to park a car. I would think if he parked his car, he would have done so where the cart road begins, because there is plenty of room right there after the guard rail ends to park.
To me it's possible if he was on a bike that the Dogs may have tracked for a short distance, but because he is pedaling on the road, the faster he is going with distance,( I would think he was giving it all he had to get out of there) leaving less raft for the Dogs to sniff. Combine that with cars driving down that road, blowing the raft away,, the dogs could have lost the scent.
The only other thing that came to mind was the Dogs tracked him on foot for that short distance, and someone else picked him up in a car, but two things with that.
1) That exposes him out on that road for people to see him. ( rather than just stay on the cart road out of sight,)
2) How did that other person know he was done doing what he did, and to come back to pick him up unless he called him. If that happened the police would have that ping.