A Beacon is a radio device used to send out a locator signal.
Cate Archer was tasked with placing a beacon in the radio room of the Freighter Lorelei after she had snuck on board. Before she could place the beacon, she was captured by Magnus Armstrong and thrown into a supply room. Before Inge Wagner had her killed, the Lorelei started sinking due to Explosives being set off. Archer was able to get out of the room and placed the beacon before she abandoned ship.
Radio beacon on Wikipedia