do french subs have backup lights and auto-surrender systems?
From an article about WWI ships ...
“Modern warships are naturally prone to developing a magnetic signature thanks to ferromagnetism. The earth projects a magnetic field, like a giant bar magnet. As a ship sails, it cuts across the planet’s magnetic field, and the magnetic domains inside the steel of ship’s hulls align and magnetize the hull.”
https://medium.com/war-is-boring/how-britain-beat-germanys-wwii-magnetic-sea-mines-bfec5558704c
A decent modern sub is made from austenitic steel for that reason. They are not ferromagnetic and don't leave that trail.