A secret shopper, sometimes called a mystery shopper is an independent contractor who does audits in retail stores with out being seen.
Marketing companies use secret mystery shoppers to go into retail stores and do market research on all kinds of different areas.
Mostly retail employees know this as being shopped. They may be be evaluated on how well they are preforming their job when they think nobody is looking. But, secret shopping is used by more than the retail stores themselves. Manufactures of all kind of products want to know how well their products are being merchandised or pushed by sales people.
An example would be, a vendor pays for a front end cap display of his merchandise in a supermarket. He needs to know if all stores are actually complying.
Sometimes they want to check on rules being complied with. Such as selling tobacco to minors.
A lot of companies use secret mystery shopping to spy on the competition. What is the competition doing better than you are? What are their prices?