Final answer:
An auditor is least likely to discover equipment retirement through the purchase return and allowance account, as this account deals with returned purchases rather than fixed assets.
Step-by-step explanation:
The auditor is least likely to learn of the retirement of equipment through reviewing the purchase return and allowance account. This account is generally used to record returns or allowances of purchased items and wouldn't normally include information about fixed assets retirement. Instead, auditors can learn about equipment disposals by examining insurance policy riders, which may be updated to reflect changes in covered assets, by analyzing debits to the accumulated depreciation account, where a reduction may indicate disposal of assets, or by reviewing depreciation schedules, which would be adjusted for retired assets.