I have been ID in two shops. The first were the popular recurring grocery store shops. They had a brief shop where you observed the front. Also, you could do a location every other week, and they had dopey questions for the deli, meat, produce and at this location, the jewelry store. So, I thought one of the front people was watching me. At the following shop, a customer asked me if I were a secret shopper as the associates were talking about me. I self-excluded the location but still shopped it weekly, and asked dopey questions (not from the shop) all three of the areas. I did this for 3 months, until it seemed the coast was clear.
Another was a mattress shop. As soon as I walked in, the employee said I was that guy who was the secret shopper. I protested, he insisted, I acted mad and he said if I were here to look at mattresses for real, then we could go ahead. I still insisted. We did the interaction. I contacted the scheduler and told her what happened. I was paid, but excluded from the clients locations. About a year later, I was still receiving emails from the scheduler. I recounted what happened. She said check the location and see who was working. Another employee. She assigned me the shop and I followed through and did it. Now I can shop the client, but do not do it more than 1-2 times a year, and haven't shopped the location since then.
Do not read so much, look about you and think of what you see there.
Richard Feynman-- letter to Ashok Arora, 4 January 1967, published in Perfectly Reasonable Deviations from the Beaten Track (2005) p. 230