The game won't give you any sort of warning on circle runners, you gotta watch out for them yourself.
I myself am both lucky and oblivious
The only circle runner I've seen was "Torpedo is a dud sir" call when the torpedoes were only 3/4 of the way to the target. Took a look on the attack map and sure enough there was a "Dud" laying next to my boat.
Lucky? It was a dud.
Oblivious? I had no idea it was coming.
Magic