It happens.
I did a shop where I was first a mystery shopper and after a small purchase, I revealed per instructions that I was there to perform the inspection, presenting the employee the letter of authorization.
When I did the "reveal" and presenting of the LOA, the employee read it and then told me in no uncertain terms to leave. He said I could not take any photos and that I must leave. The only thing I asked the employee was, "so, you are refusing for me to perform the inspection?" No reply, I was shown the door.
Okay by me! I took the required photos (off property), wrote everything down on my worksheet and went on my merry way. Later, when filing the report, I explained how it "went down".
I got paid for the shop.