Systems and methods for retrospective scanning of network traffic logs for missed threats using updated scan engines are provided. According to an embodiment, a network security device maintains a network traffic log that includes information associated with network activities observed within a private network. Responsive to an event, the network traffic log is retrospectively scanned in an attempt to identify a threat that was missed by a previous signature-based scan or a previous reputation-based scan of the observed network activities. When the threat is identified as a result of the retrospective scan, then remedial and/or preventive action is taken with respect to the threat.