Some companies, including Corporate Research International, call "Mystery Shoppers" "Auditors".
From my limited experience, a secret, mystery shopping visit is normally less than an hour at a location. The exception is for shopping related to vehicle maintenance. (As a guy, I not sure about trying on dresses.)
Traditionally and auditor would be introduced to staff. The auditor could spend one or several days at the same location.
I am now looking for auditor listings.