On June 28, 1919, the day this was printed, the Treaty of Versailles was signed in France ending the state of war between Germany and the Allied Powers of World War I. That’s the context for the “hun mine-layer” comment.
Lots of fishing boats and merchant men were used (by all sides) to secretly drop off naval mines during WW1, it became synonymous with someone who is being treacherous and secret sabotaging.
All sides would constantly accuse eachother of secretly laying mines with civilian vessels, and all sides would constantly blame the other for accusing innocent people of laying mines. It was probably a constant source of news articles.