Unobtrusive Film Production

How do they make pictures now? Large, conspicuous cameras that tell everyone - here's a picture being made. But, I think we already have enough technology to produce pictures in public without the need for extras and cordoning off. In fact, this will give rise to collectible films. The first take, one member of the public was involved - without realizing. Next take, someone else. Some of those takes will become very rare.