I think this is a reasonable possibility. I don't think the Androscoggin river is passable very far south because of the numerous dams (without locks/bypasses) along the way. He wouldn't have been able to go more than two miles downstream, and not at all upstream (from that particular boat launch). Perhaps he left the motorcycle at that boat launch, launched his boat on the Kennebec river, drove back up to Lisbon and left the white vehicle, then headed back to his boat and ditched the bike. I'm reaching, I know, but he did have I believe over 3 hours of lead time before they found his car (or maybe it wasn't there yet).