The most effective way to detect fraudulent claims for personal travel as business expenses is by reconciling claims against the business requests approved by supervisors. This method helps to verify that each claim corresponds directly to an approved and legitimate business activity, which is a critical checkpoint in detecting fraud in travel expenses.
Institute of Internal Auditors (IIA) Standards and Guidelines.

If the engagement supervisor accepts an expensive gift from management after the issuance of a final audit report, it violates The IIA's Code of Ethics principle of objectivity. Objectivity requires auditors to maintain an unbiased mental attitude and avoid conflicts of interest or unduly influenced behaviors. Accepting gifts can compromise the perceived or actual independence of the auditor, influencing decisions or outcomes of audits in favor of the gift giver.
The IIA's Code of Ethics on Objectivity.

Applying due professional care in planning an assurance engagement includes assessing the risks involved in the engagement area. An assessment of the risk of noncompliance with laws and regulations directly addresses the potential legal and regulatory exposures that could significantly impact the organization. This risk assessment helps ensure that the audit plan is appropriately focused and aligned with key risks, demonstrating due professional care.
IIA standards on planning, which stipulate that due professional care includes an appropriate risk assessment.

When a plant manager from within the organization is hired as a rotational internal auditor, the area he should most likely be trained for immediately is risk assessments. This training is crucial because understanding and performing risk assessments is fundamental to the internal audit function, and the plant manager may not have specific skills in this area despite having industry and management experience.
IIA education and training standards

When an internal auditor discovers fraud-related red flags during an audit engagement, the action that best demonstrates due professional care is to perform further testing to verify the existence of fraud. This approach ensures that any findings of fraud are based on thorough investigation and sufficient evidence, rather than premature conclusions. This procedure aligns with the IIA's guidance on due diligence and the thorough investigation of anomalies.
IIA International Standards for the Professional Practice of Internal Auditing.