1. A security device comprising a comparing unit operable to compare a sequence of representations of sensory data captured from a location under surveillance with other corresponding sequences of representations of sensory data; a generating unit, operable in response to the comparison, to generate a trigger signal; a representation generating unit operable to generate a feature vector representation of the sensory data, and an anomaly indicating unit operable to generate an anomaly value, indicating the difference between each feature vector in the sequence and each feature vector in the corresponding sequence, in accordance with the Euclidian distance between the said feature vectors and wherein the generating unit is operable to generate the trigger signal in accordance with the anomaly value.