On February 4th 1915 Germany issued the following:
Declaration of Naval Blockade Against Shipping to Britain
"The waters round Great Britain and Ireland, including the English Channel, are hereby proclaimed a war region.
On and after February 18th every enemy merchant vessel found in this region will be destroyed, without it always being possible to warn the crews or passengers of the dangers threatening.
Neutral ships will also incur danger in the war region, where, in view of the misuse of neutral flags ordered by the British Government, and incidents inevitable in sea warfare, attacks intended for hostile ships may affect neutral ships also.
The sea passage to the north of the Shetland Islands, and the eastern region of the North Sea in a zone of at least 30 miles along the Netherlands coast, are not menaced by any danger."
(Signed) Berlin, February 4th, von POHL,Chief of Marine Staff.
Source: Source Records of the Great War, Vol. III, ed. Charles F. Horne,National Alumni 1923
Mary was in Helsingborg on the 4th when the German declaration was issued and began the outward voyage one or two days later. When they began their homeward voyage it was only two days before the 18th. We can assume that anxiety was high and that they set highest speed to come as far as possible before the 18th.
We do not know what happened, but one possible theory is that Mary was intercepted by a German submarine. However, considering the calendar of the submarine war in the North Sea the probability of a submarine attack in the middle of February 1915 was rather small compared to later during the war. Read more.
The Swedish steamer Edda was intercepted two years later under very similar circumstances. Loaded with coal she left Seaham Harbour late in January 1917 but never reached her destination Halmstad, Sweden. The difference was that the crew of Edda survived and could tell their story.
Read the story of Edda