An IBNS ink-stained banknote is most likely a stolen one. Such marking by IBNS creates an obvious link to a crime. In other words, the conspicuous ink-staining of the banknote prevents that you can believe that the person offering you the banknote is the rightful owner of the banknote.
If you try to exchange such IBNS ink-stained notes at the National Bank of Belgium, the banknote will be withheld against acknowledgement of receipt and the competent authorities (police) will be involved in the further investigations. Banknotes ink-stained by IBNS are not exchanged.