Still on Patrol
There is a long tradition in the US Navy that refers to subs that never returned from their missions. If a submarine is considered lost or sunk, its official status changes to “still on patrol.”
It’s a fanciful way of saying that it will return home one day, even though everyone knows this won’t happen. However, this doesn’t mean that the navy won’t invest time and resources in finding out what happened to those submarines believed to be lost.