Semi-supervised anomaly detection algorithms can learn a classifier of sensor array response vectors as "normal" or "anomalous" using only a data set of "normal" responses. Following the definition in Ref. , an anomalous sensor array response deviates so much from the distribution of observed responses under "normal" conditions that it arouses a suspicion of some underlying problem (e.g., equipment malfunction, a leak in a pipe, etc.) warranting further investigation.