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A detector that failed to fire has at least three causes that are indistinguishable from outside the system. The signal was removed by a transform before it arrived, the threshold was calibrated on clean material and is wrong for real material, or the feature path on the device drifted from the one used in training. Two of the three are not in the model. The way to separate them is a break-map: apply one controlled condition at a time and measure under each.