I have done both banks shops and the gasoline stations.
It seems to me that with the bank shops, one is always doing a mystery shop, posing as a possible new bank customer. Whereas, with the gas stations, it is typically, a mystery shop and then reveal later.
IMO, recognition of a returning shopper is often based upon something that may stand out - an unusual accent, their appearance, etc. Therefore, with the bank shops being what they are, a mystery shopper may become recognised as a returning customer, asking the same questions over and over, thereby exposing them as a mystery shopper, not a possible new customer. I once sat through a very long session with a banker that threw my route timing off completely.
Gas stations? Even if they recognise you, it really does not matter. You are there and they cannot correct the areas in which you are mystery shopping .
That is why I only take bank shops sporadically.