People can get upset when you don't know the answer to this question.
3. Security is EVERYONE's business. Presidential Executive Order 12968 on "Access to Classified Information" states: "Employees are encouraged and expected to report any information that raises doubts as to whether another employee's continued eligibility for access to classified information is clearly consistent with the national security."
What issues raise doubt about a person's eligibility to be entrusted with national security interests? They are the same issues that were considered when you were being investigated for your clearance. These are discussed in Standards of Personal Conduct. For example, conduct that indicates disrespect for rules and regulations raises security concerns. Protection of classified information requires compliance with a complex set of rules and regulations. Willingness and ability to comply with rules and regulations is, therefore, an important qualification for access to classified information.
The excuses that people often give for not reporting are discussed in No Good Excuses. For stories of people like you whose decision to report or not to report made a difference by helping a co-worker, catching a spy, or letting a spy get away, see People Who Made a Difference.
When you report factual information about another person, your security office will take appropriate action. That's their job. If you so desire, they will protect your identity as the source of the information. If you have any doubts about this, get a commitment from the security office before reporting the information.
Related Topics: Reporting Improper, Unreliable or Suspicious Behavior, People Who Made a Difference, No Good Excuses for Not Reporting, Counterintelligence Indicators, Security and Suitability Issues, Insider Threat to Information Systems.