Lone DD's need not to take care for any precious merchants they are assigned to protect, so these guys have more spare time
to go hunting. If these two guys would have been escorting something, they would have broken up their attack to go back to their assignment - looks kinda bad if they spend hours on end going for a sub, only to have their merchants been sunk a couple se miles away...
Usually I leave them alone, unless it's a
single ship,
and I am in dire need for results to report back in port to my superiors.