According to an article in The Washington Post [1] he does not trust banks. He did not normally keep it in his car; but,"had it in the car with him because he was planning to look for homes closer to his children that weekend, and his parents were scheduled to be out of town." (He normally keeps hte cash in his parents' house.)

[1] https://www.washingtonpost.com/national-security/stephen-lar...