Expense reports generally have a two-stage verification process. Employees submit expense reports which are then approved by managers. Managers are supposed to check that each expense is valid before approving them... but managers are pressed for time... so they routinely approve expense reports and trust that their employees are doing the right thing.
Unfortunately, expense report audit is still a manual process. Auditors verify receipts against expenses to ensure that expense claims are legitimate before approving the report for payment. With the large and growing amount of expense reports, receipts, and other information that needs to be reviewed, auditors generally limit their verifications to matching receipt amounts to expenses claimed. In fact, it is so expensive to do even this basic audit that organizations typically resort to random sampling expense reports for audit or setting thresholds for expense amounts to identify which expense reports should be audited.