There's an analogue method too - just do a very long exposure of a busy street. There is so little reflected light from people (relatively speaking) that it barely registers on the film/sensor, hence they are 'invisible' in the final shot. That's why old street photos are sometimes empty of people, because both lenses and film were slow, leading to long exposures.
That's interesting to me, for I (a non-native speaker) am well familiar with the word 'cagey' from more contemporary movies and books. The dictionaries I checked list etymology as unknown, however. I'll see if I can't ask one about the "K.G." hypothesis.
