How do you know if cash euro bills you are using are not secretly marked by government for tracking money flow?
Lol. They are marked with serial numbers, and I just assume there's some level of tracking already.
This was used many times in history in different countries with UV ink markings, but more likely they are using some different tech this days.
You mean RFID and NFC? Nothing a Faraday wallet can't stop, until you spend them.
I even saw several websites and apps for tracking euro bills.
I've seen a site years ago that tracked how banknotes spread through different countries. But none of that matters, as long as your money is fungible.