But do we even know if it was scrap from the job? It could have just been things he may have decided to scrap. I've seen people that drive around looking for metal in people's garbage. All we know is "minimal transaction" but nothing on what it actually was.
Maybe that was the time lapse between leaving work and getting to Arrow, he was picking up scrap some where. Odd that he left early that day and didn't call or text his wife. To me that is a red flag that he may have been planning something.