The possible hitchhiking is beginning to sound like a very real possibility to me. I am sure that if he woke up after his dad left, he would have probably been bored very quickly, so perhaps he set off to hitchhike to his friends' house taking his fishing pole with him, and somebody came along and coaxed him into their vehicle.
But assuming that he tried to hitchhike and wasn't seen by neighbors walking down the road... that's not too far fetched, to me. I don't always see who walks down my road and I live in a rural area. When people are walking, they don't make that much noise. Unless you're sitting by the window and watching out, you might not notice a person walking by. He could easily have not been sighted at all before getting into a vehicle with someone. That person could have made certain not to pick him up right in front of another house. I don't know how far apart Mark's house is from other houses, but if you live in that area, you might not always know what's going on around you every minute, right?
People still hitchhike in the area I live in. I see them all the time. Kids 13 years old may not even think of the dangers of accepting a ride from strangers, and especially from someone they even remotely know.
It is a possiblity, IMO.