One time I got a car detailed, and when I was paying at the end, I noticed that the employee was writing out a receipt and marking that I had paid with a gift card. I don’t remember if I was paying with cash or a credit card, but either way I figure some sort of tax evasion and/or money laundering scheme was happening.

Probably skimming the till.

They return a previous customer’s transaction a refund to gift card, then take your cash and pocket it. There’s a ton of grifts like that.