Is there really a secret? It really depends upon the way you look at it.
Very few know that up to 500,000 people in the UK are registered as Secret Shoppers, (or Mystery Shoppers as they are known), but in reality less than 10% of those are regularly dedicated to the task. So it can be regarded as a specialist field.
On the other hand, Mystery Shopping does exactly what it says on the tin. And the clue of course is in the name. A shopping assessment of a service is carried out by an unknown person, measured against precise criteria. The specific instance is the only secret. And there's a lot of planning, preparation and skill in getting it right.