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RBI has a whistle blower cell. They are involved in reporting the money laundering and hoarding cases. They keep an eye on the banks as well the deficits that come up due to cash crunch. For example, if 10 crore INR was sent to xyz bank in last week, why only x number of atms and x number of people were able to withdraw the money even when there's a limit imposed.
Whistle blower wings are active mostly in urban areas. Along with them the I-T department receives data inputs from police, common people and bank employees as well as from the purchases made by a person (you won't go unnoticed if you buy a costly item, I-T dept receives details from the retailers and malls too). You can act as a whistle blower by sending input to the RBI.
As you can see, they have a data pool, they identify the targets, trail the money which is easier as of now since the currency is new and what happens next, you already know. The question is why NOW? because earlier government did not have enough will power to take action against politicians and corporates. The political pressure prevailed upon I-T dept and hence such moves were next to impossible. Now there's more duty and less pressure. RBI whistle blower cell hasn't been active to the extent we see today.