Which of the following is NOT a common symptom of a security incident?

The presence of closed or filtered ports does not typically indicate a security incident on its own; rather, it is often part of standard network security practices and configurations. Firewalls and intrusion prevention systems often filter or close certain ports to protect the network from potential unauthorized access or attacks. While an incident might result in changes to network port statuses, merely observing that certain ports are closed or filtered does not necessarily point to malicious activity or a security breach.

In contrast, modified files or folders, alarms triggered by intrusion detection systems (IDS), and suspicious log entries are all critical indicators of potential security incidents. Modified files could indicate unauthorized access or tampering, alarms from IDS signify detected anomalies that could be malicious, and suspicious log entries can indicate unauthorized actions or attempts to breach security protocols. Thus, the option regarding closed or filtered ports stands out as not being a common symptom of a security incident, making it the correct choice in this context.