Which OT devices does passive monitoring miss?
Devices that do not talk on a segment you mirror. That includes devices that are silent most of the time, such as a spare controller or a unit that only answers a master you do not see; devices on serial fieldbuses such as Modbus RTU or Profibus; devices behind an unmanaged switch whose traffic never reaches the mirrored link; equipment that talks only over its own cellular router; and remote sites without a sensor. Passive monitoring also sees less inside encrypted protocols, and it cannot read a firmware version that the device does not announce. Close those gaps with a walk-down, the engineering project files and, where the plant accepts it, a carefully planned query with the vendor's own tools during a maintenance window. NetSenX is passive only, so its network map shows what communicates on monitored segments, and nothing more.