What does it mean?

#238910

That means that the target was a submarine and it snuck out of the target zone without your destroyers noticing.

Only naval bombers on patrol can see a sub in real time, so without a naval bomber to provide vision the destroyers will continue to fire on the last known position.

I would assume that the enemy sub was there attacking something so it was visible and when that unit of yours was destroyed the sub left or at least moved to avoid the destroyer fire.

It is possible that the sub is still there taking fire and damage, but you can't see it.