I've never done these type of shops (or even read the guidelines) but I'm wondering if it is important that the cashier not know what you are doing. I can understand if it is at a store that you do a grocery mystery shop, for example, you wouldn't want employees knowing that you are a mystery shopper. But isn't the credit card processing company the one that is wanting the info about the terminal? If the cashier sees you taking pictures of the terminal s/he might think it is odd or might think you are a mystery shopper but does it matter? After all, you aren't 'shopping' the cashier, you are 'shopping' the credit card processing.
I know shoppers aren't supposed to be blatant about it but wouldn't it be okay to just take the pics, act like it's normal, and continue with the transaction? Or is my thinking way off? I've been reading this forum for months and I'm still trying to figure things out!!
By the way, thanks to everyone who has posted because I've learned a lot!