I learnt something useful on holiday at the Cap d'Agde.
There is a map of Europe on one side of a Euro note, with an alphabet letter followed by a long printed number. Presumably the serial number. The letter is the country identification code. If you are the owner of a note with the letter Y; you may wish to pass it on.
Example of a code number on a French 50 Euro note; U05189191934.
Considered safe by the naturists;
I was advised to avoid;
I hope this is useful,