I'm sure this has been answered, but even if there were 5 Walmart stores there, she claims she bought and returned the gas can to the same store. Her receipt shows where it was purchased, so that's the one that was checked.
I also read an earlier post that suggested that in 2008 there was only 1 Walmart in Salinas. I would think if the defense could prove that she returned the gas can to another Walmart anywhere in the US they would have done so when the Walmart person testified for the prosecution Not to say that Nurmi couldn't just throw that possibility out there in his closing to try to cause doubt.