Disclaimer: I don't own Supernatural. If I did…well, we won't get into that.
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They're halfway to Kansas before Sam realizes he left his wallet back at the diner. When he tells his brother, Dean just sort of shrugs it off—tells him to get another license and a set of credit cards out of the glove box.
But Sam makes him turn the car around and go back. Even though it's possible the wallet won't be there. That somebody walked off with it for the eleven dollars he'd stashed in the billfold. But he had to go back and see. His license was in there—his California driver's license with the name 'Samuel Winchester' and his real birth date on it—and a picture of Jess. It was his favorite snapshot of her, taken by a friend when she wasn't aware she was being photographed, and he'd cut it down so it would fit in his wallet, just behind the driver's license.
He doesn't want to think of leaving her in the bottom of a diner's cash register drawer in Missouri…especially since he's having a harder and harder time recalling her face without the help of that little photo.