Ask if there is any trace of contamination or foreign materials adhering to the distance sensor area of the front bumper.
Ask if distance sensor area of the front bumper was frosted during driving or if vehicle was driven in snow.
Ask if distance sensor area of the front bumper was temporarily fogged. (Windshield glass may also tend to fog, etc.)
Is any of above conditions seen?
Explain to the customer about the difference between the contamination detection function and the indication when the malfunction is detected and tell them “This is not malfunction”.