How did Hitler learn of Churchill's plan to mine the Narvik fjords?

In the late hours of 16 February 1940, the Royal Navy destroyer HMS Cossack entered Norwegian territorial waters. There it boarded the German ship Altmark. The Altmark had served as a supply ship for the pocket battleship Admiral Graf Spee and was trying to return to Germany with prisoners of war captured by the Graf Spee, illegally using Norwegian waters to evade the British blockade.

Hitler saw in the episode a clear sign that the Allies were also willing to violate Norwegian neutrality, and became even more determined to invade Norway. The whole affair led the Germans to speed up their plans to invade the country. High command had initially thought that a neutral Norway suited their interests. Großadmiral Erich Raeder, on the other hand, argued for invasion.

Churchill was a strong advocate for action in Scandinavia, since he wanted to cut off Germany's access to Sweden. In January 1940 a less ambitious plan was first approved: mining the southern approaches to Narvik. In March the idea came up again. Hitler signed the official approval for the invasion and occupation of Denmark and Norway on 1 March. The German invasion began on 3 April 1940, when the disguised supply ships started putting to sea ahead of the main force.