Ink-stained banknotes

The ink on these banknotes is from anti-theft devices, also known as intelligent banknote neutralisation systems (IBNSs), which are activated when criminals open a protected cash container, such as an ATM or a safe in a cash transportation vehicle. These systems make stolen banknotes unusable and worthless, thus reducing the risk for retailers, banks and other professional cash handlers of becoming victims of crime.

How to recognise a stolen ink-stained banknote?
When a banknote is stained by an intelligent banknote neutralisation system (IBNS), the security ink soaks into the banknote and leaves traces which are normally more pronounced on the edges of the banknote. The colours of the most commonly used security inks are bright violet, green, blue, red or black. The ink usually flows from the edges towards the centre of the banknotes and leaves a characteristic pattern. Sometimes, the chemicals that criminals use to wash the banknotes when trying to remove the ink can change the colour. As a result, the banknotes' original colours could also be altered, and some security features may be damaged, or may even disappear.

Does it mean that all ink-stained banknotes are stolen?
No. If there are very light stains or a couple of small marks on the banknote and the edges are intact, then the marks are most likely accidental, caused for example by a pen dripping ink.

Kavan adds:

To read the complete article, see:
Have you ever come across a banknote covered with ink?(https://www.ecb.europa.eu/euro/banknotes/ink-stained/html/index.en.html)