The most important factor for the organization to ensure when reducing the retention period for media containing completed low-value transactions is that the retention period complies with data owner responsibilities. Data owners are accountable for defining the retention and disposal requirements for the data under their custody, based on business, legal, regulatory, and contractual obligations. The policy should reflect the data owner's decisions and obtain their approval. The policy should also include a risk-based approach, but this is not as important as complying with data owner responsibilities. The retention period should allow for review during the year-end audit, but this may not be necessary for low-value transactions that have minimal impact on financial reporting. The total transaction amount may have some impact on financial reporting, but this is not a direct consequence of reducing the retention period. References:
CISA Review Manual, 27th Edition, pages 414-4151
CISA Review Questions, Answers and ExplanationsDatabase, Question ID: 255

A CO2 system could be a concern for an IS auditor when used to protect an asset storage closet. While CO2 systems are effective at suppressing fires, they can pose a significant safety risk to personnel. In the event of a fire,the CO2 system would fill the room with carbon dioxide, displacing the oxygen. This could be hazardous to anyone who might be in the room at the time12.
References: ISACA's Information Systems Auditor Study Materials1