HMS SEAHORSE (Lt D S Massey-Dawson) is believed to have been lost with All Hands on 7 January 1940. At the time of her loss, it was thought that SEAHORSE may have been mined.

After the war, research was carried out in which German records were examined. It was concluded that SEAHORSE had been depth-charged and lost on 7 January 1940 by vessels of the German 1st Minesweeping Flotilla. However, this attack has since been ruled out as the cause of her loss following detailed examination of all relevant German records, including the flotilla diaries.

Her movements have been reconstructed as far as possible using her patrol orders and the information given to the Commanding Officer before sailing. This research has established that there is a possibility that SEAHORSE could have been caught in an anti-submarine minefield in her patrol area, but which was unknown to the Admiralty at that time. There were no survivors.