As I looked at the pics of the Datsun on set I had a few different thoughts come to mind as to why it's there. I think Sam is trying to impress the hot blonde at his work (Rosie). On his way to work he yells at Bumblebee like he did in the start of ROTF telling him not to do anything crazy, stupid etc he's trying to impress this girl. Bumblebee clearly not taking Sam's crap scans this Datsun. Sam brags about how awesome his car is to Rosie all day at work. Once the work day is over this P.O.S Datsun pulls up putting Sam in a pretty funny spot trying to explain his Datsun to Rosie. I may be way off but it sounded logical and kind of funny.