The -18 °C limit in the Quick-Frozen Foods Communiqué applies to the product itself, not to the cabinet air. When the door opens or defrost runs, the air warms within minutes while the product warms slowly; the peak of that slow warming decides how low the setpoint must go.
An alarm threshold set just above the setpoint sounds at every door opening, and soon nobody looks at it. Adding a time condition, for example alarming only on a rise lasting longer than 30 minutes, keeps it meaningful.
The Codex requires air temperature to be recorded regularly with an EN 12830 compliant instrument and the records to be kept for at least one year. In retail, cold rooms under 10 m³ and sales cabinets need only a clearly visible thermometer.