The internal audit activity can promote continuous improvement of organizational controls by facilitating control self-assessment (CSA) sessions. These sessions involve managers and staff in evaluating the effectiveness of controls within their areas of responsibility. This approach not only promotes ownership of controls among process owners but also helps identify areas for improvement and fosters a culture of continuous improvement. By engaging managers in these assessments, internal auditors can help ensure that controls are understood, effective, and continually refined based on feedback and changes in the risk environment.
IIA Practice Guide: Control Self-Assessment
IIA Standard 2130: Control

According to the IIA guidance on using the maturity model for assessing an organization's risk management program, a key activity to examine is "Monitor and Review." This element evaluates how well the organization continues to monitor its risk environment and reviews the effectiveness of risk management strategies over time. It is crucial for ensuring that risk management adapts to changes and continues to align with organizational objectives.
Institute of Internal Auditors (IIA) - Guidance on Risk Management Maturity Models

Given the unusual observation in the travel expenses reports, the most appropriate next step for the internal auditor is to investigate whether there are any special arrangements regarding senior management travel. This investigation could reveal explanations for the absence of typical travel-related expenses, such as pre-paid packages, special corporate arrangements, or even potentially unreported expenses, which could be critical for compliance and ethical standards.
Internal Auditing Standards on performing audit work and investigating anomalies.

The strategy for professional development that best demonstrates an internal auditor's competency is the creation and adherence to professional development and training plans. Such plans are tailored to the auditor's needs and goals and include a variety of learning activities and programs designed to maintain and enhance their auditing competencies. This comprehensive approach ensures continuous improvement and relevancy in the profession.
IIA's guidelines on Continuing Professional Development and standards for maintaining competency.

Segregation of personnel duties is a control that is designed to prevent an undesired activity from happening, such as errors, fraud, or misuse of resources. It means dividing the tasks and responsibilities related to a process or activity among different people, so that no one person has complete control over it2. This reduces the opportunity and incentive for anyone to manipulate or falsify the data or transactions, and increases the chances of detection if they do3.
References:
1: Preventive Controls: What Are They & Why Are They Important?3
2: Segregation of Duties - The Institute of Internal Auditors or The IIA
3: Segregation of Duties - Wikipedia