Anti Corruption investigation against retired public servant

My Father was a government servant for all his life. Had no complaint ever against him during his service. After he retired he was also given extensions for 3 years. During this time, someone sent an anonymous complaint against him to ACB Gujarat. They investigated and reached-out to all the places for bank accounts, properties and so on. My dad provided all documents whatever was asked for. They also went to office of one of my sister. This was in 2017-18. Then my dad was called to file his response. He did respond to all their questions, Essentially they found nothing as we have very limited property. After filing the response the officer there said, that there is nothing in this complaint and should be closed. Now, after two years (2020) they have called again to get some more documents I mean how long will they keep asking? When does this end. Can we do something about this?