Well, the guy happened to be watching him because he was wondering if the driver who had to wait (Scott) was getting impatient, and then he noted the smile from the side. Not posing, IMHO, just content that his diabolical plan was working. He wasn't in the hour for more than an hour. The guy remembered the time because he had just finished his lunch break. He couldn't pick Scott out of a line-up, but he definitely remembered the truck and boat.
Modesto is right off of Hwy 5, so I'm thinking 7-8 hours at the most from San Diego. Of course, the roads are busy for the holiday weekend. Plenty of time for Scotty to talk....
Oh yeah, I wonder if he spoke to his attorney during that ride..or did he keep his mouth shut. He doesn't really strike me as the type of guy to share anything beyond small talk..BUT he might be so nervous he spilled his guts.