Visitor logs would be the best additional method for identifying individuals who enter the building in the event of a camera system failure. Visitor logs track who enters and exits a secured facility, providing a record that can be cross-referenced with security events, like the discovery of a suspicious flash drive. In this case, reviewing the visitor logs could help identify the vendor representative who dropped the flash drive. CASP+ highlights the importance of physical security measures, such as logging and auditing access to facilities, to complement digital security controls.
References:
CASP+ CAS-004 Exam Objectives: Domain 2.0 - Enterprise Security Operations (Physical Security and Access Control Logs) CompTIA CASP+ Study Guide: Physical Security and Incident Response Procedures