In risk management, it is standard practice to document risks to the extent that they are identifiable and reasonably foreseeable. If an undocumented risk is realized, the risk manager should address the concern by explaining that risks are documented to the practicable extent possible. This means that while every effort is made to identify and document risks, some risks may not be captured due to their low probability, lack of historical precedence, or other factors.
PMI's guidelines on risk management recognize that not all risks can be foreseen and documented, and risk management processes should focus on those risks that are most likely to impact the project.