MUMBAI: In the next fortnight, the Reserve Bank of India RBI ) will have to deal with one of the best kept secrets of demonetisation — the value of banned currency notes which never came back to the central bank.Till now, the Centre and the central bank have not spelt out the quantum of delegalised bills that individuals and businesses neither exchanged with banks and RBI nor deposited in bank accounts.But, they would soon be forced to end the currency mystery. The monetary authority will hold a meeting with its auditors later this month to put a number to the notes which were not returned, according to a person familiar with the matter. The meeting assumes significance because in the absence of the ‘missing notes’ number, RBI cannot finalise its balance sheet for the year ended June 30, 2017 and declare dividend — or, surplus — to the government.According to the RBI accounting policy, ‘currency notes in circulation’ are considered as a liability relating to the central bank’s issues department. Every currency note, of all denominations, black or tax-paid, is the liability of RBI, as captured in the words printed on a bill— “I promise to pay the bearer the sum of X rupees”. While the currency is treated as liability, bonds and foreign exchange constitute assets in the central bank’s books.Thus, to freeze the liability (as on June 30, which is the year-end for RBI), the central bank has to add the lower currency notes which were not demonetised to the newly printed currency (through remonetisation that followed the note ban ) and net it with the currency that were returned to banks and RBI.Currency bills having denominations of Rs 500 and Rs 1,000 ceased to be legal tender from the midnight of November 8. Collectively the value of the demonetised notes was put at close to Rs 17.5 lakh crore and accounted for more than 85% of the currency in circulation.In the months following the dramatic announcement, there was a growing perception that an unexpectedly high amount of demonetised notes has flown back to the banking system — with the holders either choosing to deposit undisclosed cash to pay tax and come clean, or launder cash using the services of an overburdened banking system. While the government claimed that demonetisation has significantly widened the tax base, Opposition parties argued that it has failed to shrink the black economy as most of the demonetised notes came back to the system.Even as the debate raged, the value of demonetised notes that were not returned was not revealed, despite queries from the media and the Opposition. According to money market circles, this number could be around Rs 25,000 crore.“It is not possible to physically count. So, the auditors are expected to discuss the audit procedure for arriving at the number and verifying the figures submitted by the department concerned.Besides, there are other issues like the quantum of notes with the central bank of Nepal that has to be estimated,” said another person.The RBI spokesperson did not respond to a text message from ET.