The Secret Of The Riviera

Story 3 of 7 in The Seven Secrets of Marcus Orlando series, published in The Story-teller, February 1934. Reprinted as The Secret at Sainte Marie Sur Mer in Secret Services, a collection of espionage stories by Frankau, 1934.
Marcus Orlando is on vacation in a small community a short distance from Cannes and Monaco. On a pleasant hike along the coast, he comes across a piece of paper he is certain is not just detritus, enough so that he alerts French Intelligence as he digs into a mission to undermine the Monaco government.