sneakers I work for the same companies you work for I'm sure, as far as mystery shopping companies go. Maybe there are a few more or less shops in my area than in your area.
You have to ask yourself some basic questions. As an example: Business are popping up every day. So how does the IRS know they are a legitimate business? Just because a business gets their EIN number doens't necessarily equate to legitimate business. What if that business doesn't pay any tax or doesn't file their taxes in a given year?
Works the same way for "non profits" What is the "non profit" doing with their money? How are they getting their funding?
Are they registered with the state? Just because they have a tax exempt status with the IRS does not mean they are "running a straight ship"
So who does the IRS send in to find out these things? They send someone in who will not "scare" the business or non profit. Someone who can assess and come off as just an ordinary person asking a few questions. Walk in the door say something like "hey I just noticed your open for business. Mind if I look around?" or "I'm interested in learning more about your organization"
There are many, many questions you can ask yourself then do some searching and find some pretty unique adventures.