A method of verifying a triggered alert in a man overboard detection system comprises: receiving (600) a plurality of sequential frames of image data associated with the triggered alert followed by motion compensating (616) difference frames formed by generating differences (608) between successive frames. A summation of the motion compensated difference frames is then generated (618). The summation image is then analysed (630, 632) and the detection associated therewith is classified (634, 636) in response to identification of a cluster of pixels corresponding to a predetermined size range.