In the US, mystery shoppers were all the rage- twenty years ago. Now, high-level and specially trained shoppers do revealed audits quarterly that last for hours in a day. In my experience, three is about the average. There was very little mystery. Stores know when their inspection window is open, but not what day or time the visit will occur. But any employee working the drive-through memorizes the license plate and physical descriptors of the known shoppers! Lol

The practice in the US isn't gone, but has changed form and become extremely specialized work. I am easily more rattled by a visit from a brand inspector than I am from any health inspector. Or I was when I managed restaurants. It is much less nerve-wracking to be the one performing the inspection!