Pg. 1: "In order to operate in accordance with our ethical principles and at the same time act as a strong and responsible partner towards our customers, AUMA Riester regularly carries out on-site audits and written surveys of suppliers in order to monitor compliance with our standards and the statutory requirements. The audits are announced at short notice. Based on our risk analysis, we select susceptible suppliers for inspection. Where weak points or violations are discovered during an inspection, the supplier must prepare a plan for corrective measures and remedy all violations within an agreed period."

The paragraph mentions on-site audits of suppliers, but doesn't mention a third-party auditor, which makes me think it is in-house/self-reporting.