My oversimplification/takeaway: When false negatives are very rare and their costs very high you can't rely on an empirical assessment of a detector's accuracy, it will be near 100%. Because the costs of false negatives are so much higher than the costs of false positives you could focus on false negative rate, but there's not enough data points and the costs of getting a single false negative are too high to estimate this empirically. You need to audit the procedures to determine if the detector can reasonably detect the thing you are interested in.