Marcel and Sarah

Who were they? Why did they leave a red padlock on the Greig Street bridge? Was it a suicide pact, or just a token of their love? While pondering this, a young French couple asked me to take a picture of them. The chap handed me a heavy metal Canon SLR, with manual focus. Cripes, I'd better get this right, I thought. They were delightful, but it took me no nearer to solving the riddle of Marcel and Sarah.