how did walt and billie learn chris was in california

Walt and Billie received an unpaid ticket in the mail, which alerted them to Chris' presence in California.

On August 10, shortly before meeting Jan Burres and Bob, McCandless had been ticketed for hitchhiking near Willow Creek, in the gold-mining country east of Eureka. In an uncharacteristic lapse, McCandless gave his parents’ Annandale address when the arresting officer demanded to know his permanent place of residence. The unpaid ticket appeared in Walt and Bil-lie’s mailbox at the end of August.