Not always. They're been scanned for fakeness. At least that's the official statement. But who knows...

Neither scanned nor checked for serial numbers.

I was at a Döner place where to pay cash, I had to feed the cash to a machine. I'm pretty sure it's scanning the notes for more than being fake. At supermarkets the ever more popular self-service terminals probably process most of the payments by now also might scan whatever. You can still opt out...

Happened to me in Eastern Europe very recently

This is part 1 https://netzpolitik.org/2025/bargeld-tracking-du-hast-ueberwachungsinstrumente-im-portemonnaie/#netzpolitik-pw Part 2 is linked at the end. I’m pretty sure you know how to translate these articles 😉

The security company that transports the cash from the supermarket to the bank probably scans all the notes again. They would have a large enough size to make a reasonable business case out of that data, I assume.

Not very common here (countryside of Brazil) It used to be common in my infancy, they used to check the note's watermark. But that was at a time where a 100 BRL bill was a lot of money (they used to be rare and you would be super badass to hold one). But nowadays they are worth less than $20 😥