The French Key.

Private detective Johnny Fletcher and his sidekick Sam Cragg skip out on their rented room, but when they sneak back to retrieve their luggage, they discover a dead body on the bed, holding a gold coin in its hand. Fletcher is told by a coin collector that the piece is an old and valuable Spanish coin, but Fletcher soon begins to suspect that the man is himself involved in the murder. Fletcher's investigation leads to he and Sam getting caught up in a murder and gold smuggling scheme.