many many many US ships met their fate in this very same way off the US East Coast during late 1941 early 1942.
Historically, nearly every ship that traveresed the East coast during the time sailed with all lights on as in peace time. It was unheard of for an attack to be mounted in US waters that close to the coastal areas.
It is likely that the ship was modeled after historical accuracy and never even saw you.
Your engagement with the American merchant is pretty much the par for the course as far as historical accuracy goes in 41-42. many attacks on US Shipping in the area were so text book the germans might have well just been shooting at training targets!
but dont count on much luck in the American waters after april or may of 1942... the yanks were behind the brits in ASW tactics right up until they wised up in mid 1942. air patrols and ASW shipping will become more numerous.