If you have an illegal dwelling in your building, the inspectors will get in, at some point, and the risk to the homeowner in fines and penalties is too severe to pretend that the violation will not be written because there is only one anonymous complaint. Your own tenant may have turned you in or a neighbor, and the complaints will continue until you comply with the law. The risk to life and to property is too severe for this problem to disappear on its own.
Any landlord in New York City should be familiar with the potential high cost in evading the laws relating to legal conversion of existing dwellings and in creation of multiple dwellings without prior plans and city permits.
Just check out this schedule of penalties that may be imposed for violation of the city's code: