I referenced my mystery shopping business in my resume. My business is in performing business evaluations and audits. I didn't say "mystery shopping." because people tend to take that as a joke.
I stated that I am proficient in Excel, Access, and have experience performing inventory checks and audits.
I didn't say "mystery shopping" because I work for a company who handles sensitive, expensive, cutting-edge information, and I didn't want them thinking I was a corporate spy, and good at it!
Once I was hired though, I mentioned mystery shopping when asked. My boss is now sometimes my guest on dinner shops, I take clients on dinner shops and tell them they need to check out the bathrooms, and that they have to order alcohol even if they don't drink it. I always clear it with them beforehand, but... you'd be amazed at how these rich, powerful people with businesses of their own get a kick out of things like remembering the server's name or proudly reporting back how clean the bathroom was.
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Plan the work. Work the plan.