According to the Roosevelt Island Operating Corp (RIOC) Public Safety Daily Incident reports:

10/11/20 2050 400 Main Street (platform) Public Lewdness PSD responded Referred to Transit PD.

RIOC Public Safety Deparment Chief Kevin Brown adds:

The public lewdness complaint occurred on the lower level of the F train. A complainant walked into PSD and stated she witnessed a male exposing himself. PSD notified the transit police and responded to the location. Upon arrival not both NYPD and PSD the male was gone on arrival.