Frank L. Packard 
The Adventures of Jimmie Dale 

Frank Lucius Packard (February 2, 1877 – February 17, 1942) was a Canadian novelist. He was born in Montreal, Quebec and as a young man went to work as a civil engineer for the Canadian Pacific Railway. His experiences working on the railroad led to his writing a series of mystery novels, the most famous of which featured a character called Jimmie Dale. Jimmie Dale was a wealthy playboy by day, but at night put on a costume and became The Gray Seal, who enters businesses or homes and cracks safes, always leaving a diamond shaped, gray paper 'seal’ behind to mark his conquest, but never taking anything. He was just doing it for 'the sheer deviltry of it’ at first, but when a woman catches him she blackmails him to war on certain crime organizations.

Great story of a good guy battling crime in his own way. Story ends well.