Maybe he was going to return it - like the terrorist that tried to get his deposit back on the van used in the first World Trade Center truck bombing?
Seems really odd for him not to have any ID on him, but he had a week-old receipt on him. BTW - how did they figure out who he was so quickly? Maybe registration or ID found in his vehicle??
Good memory re rental van.
Maybe he intended to meet up with someone regarding the receipt.