Mystery shopping causes losses
Mystery shoppers focus on the behaviours they have been sent to inspect for. Because these behaviours bear no relation to the variety of customer demands they can cause sub-optimisation through wasting time, saying things to customers that customers have no interest in hearing and spoiling the relationship with customers. That can only lead to further losses.
A further form of sub-optimisation is the impact this kind of inspection has on morale. When the only joy you have at work is spotting mystery shoppers…