Generally the shops, as they are called, are done during a specific date range, and in some cases, a time range. For example, a company may hire a mystery shopping company to perform 10 mystery shops during the first week of October between the hours of nine to five.
One of the odd things which has been occurring is some businesses are actually training their employees in what to look for in a mystery shopper such as the type of questions they have. Some blatantly get the questionnaire the mystery shopper is going to fill out and train from that.
So I go back to my question, are you defeating the purpose of the mystery shopper? My answer is yes. I think of it as cheating, here's why. If the employee only does what is required to do because you have trained him for the mystery shop, then the treatment the shopper may receive may be scewed.
If the purpose of the mystery shopper is to evaluate behavior, then it defeats the purpose if a business trains for the shop instead of training the employee(s) on what is required service for all customers. Further, by telling employees the time frame the shops may occur is again problematic.
And, a final thought, if an employee IDs a mystery shopper, that means that shopper gets fired from ever doing that shop again, regardless of the State. The weird thing about this is that shoppers who have been identified, was generally due to negative feedback, which is also telling.
The job of the mystery shopper is just to evaluate how things are at the time they go into an establishment. They are not the enemy. Companies spend millions a year on this service, but it could all be money down the drain since some prefer to cheat by warning employees and/or outing mystery shoppers.