Probe A was one of three probes launched from the USS Rio Grande 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 magnetometer, 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 A, on its approach to the third planet in the system, detected a .0012 fluctuation in the hysteresis curve coming from the sole moon of this planet. It was later confirmed that the curve originated from the crash landed Yangtzee Kiang. (DS9: "Battle Lines")