Which of the following is considered a reportable activity in personnel security?

The identification of unexplained affluence as a reportable activity in personnel security is based on the principle that such financial behavior could indicate potential issues related to integrity and trustworthiness. Unexplained affluence refers to an individual exhibiting a lifestyle or financial status that is not commensurate with their known income or financial background. This disparity could raise red flags regarding possible insider threats, such as bribery, coercion, or illicit activities, and necessitate further investigation to ensure the individual's reliability and suitability for access to sensitive information or positions of trust.

In contrast, attendance at security training sessions, signing non-disclosure agreements, and participating in team-building activities are standard practices in personnel security that do not inherently signal any concerns or require reporting. These activities are generally seen as part of routine operational procedures and do not indicate potential risks or vulnerabilities in the same manner that unexplained affluence does. Thus, it's crucial to monitor signs that may indicate a need for further scrutiny, and unexplained affluence is a significant flag in the context of personnel security.