During the latter half of the voyage the ships radio operator had heard messages from a British cruiser which judging from the transmission strength had been keeping station with "Gulflight". At a point west of the Bishop Rock lighthouse, Scilly Isles, at 11 a.m. on 1 May, "Gulflight" was challenged by two British patrol vessels, HMS "Iago" and HMS "Filey", which queried her destination. The patrol ships had been searching for a submarine which had been sinking ships in the area over the last couple of days. The patrol vessels were not satisfied with "Gulflight's" papers and suspected her of refuelling the U-boat, so ordered the tanker to accompany them into port.