Having been forced to abandon his car in the Detrital Wash after a flash flood, Alex then leaves it to go adventuring into Mexico. When he re-entered the United States, however he goes back to Detrital Wash, seven and a half months after he had originally left his car. However, we were told that Alex had buried some of his belongings that he might want later, and at this stage he unearths some of these. Note what the text tells us about this:
The Park Service had long since impounded the vehicle, but he unearthed his old Virginia plates, SJF-421, and a few belongings he'd buried there.
It is interesting why Alex should choose to do this after having made such a statement by burning his documents and ID cards. This act indicates that he was by far the detached individual that he thought he was or pretended to be, and that certain elements of his past and identity still had immense importance for him.