It's possible they caught his vehicle on video around the Mennonite community and figured out he was the owner. They may have seen something suspicious enough on video, or found some other probable cause with him that would allow them to obtain a court order for the geo data from his cell phone or vehicle. That and/or other surveillance footage could've allowed them to retrace his movements.
I wonder what initially brought him to their attention. Was it just that his car had been spotted on surveillance near the compound?