I first encountered the concept of "audit trails" in an ISO 9001:2000 Transition Training Course. It was a scheme whereby the audit was divided into five audit trails, each covering a grouping of processes and tied to the various clauses. To oversimplify, one trail is "product realization" and would cover the flow from order entry through shipment and include procurement, design, planning, testing, calibration, control of nonconforming product, shipping.
I don't know how much of this information is available outside of course materials, but this is the gist of the concept.