Before I started working for the railways I used to do mystery shopping and similar to keep me going through uni, and I can genuinely say my experience from it has helped develop my customer service skills, as you get to see the really bad, and the really good.
Railway-wise I've done stuff on behalf of Network Rail (Major Stations) and ATOC, and outside those I've done stuff for various high street chains, restaurants, pubs, and later on I focussed on age verification jobs, where generally I'd get given a list of stores in a given area that needed auditing and be paid to travel down there and do them all in one go. Occasionally you'd pick up a Wetherspoons or Nando's job too on the way back so dinner was sorted too
Some job specifications make it so obvious you're a mystery shopper that you might as well not bother, but with most you're generally allowed a bit of flexibility to adapt and play it out a bit. Ironically, the only time I clocked that the staff thought I was a mystery shopper was when I was genuinely in the shop in question to buy Toothpaste...