Probe C was one of three probes launched from the USS Rio Grande in 2369 when it was scanning an unnamed star system in the Gamma Quadrant for the missing USS Yangtzee Kiang. The probe was equipped with a differential magnetomer, so it could pick up a fluctuation in the hysteresis curve. This curve, caused by the interaction of the magnetic field with metallic composites in the Danube-class hull, would be used to find the Yangtze Kiang in the system.
Probe C, on its approach to the seventh planet in the system, detected no change in the hysteresis curve when it scanned the planet. (DS9: "Battle Lines")