What happens if the hole in the piece of card, blocking out all other paths, is very small?
Arrange the waypoints so that the only paths that can be explored pass through a narrow gap (a slit in two dimensions). Drag the detector up and down. Notice the changes in brightness.
This effect is a surprising prediction—putting a narrow gap between source and detector results in an interesting spread of brightness at the detector. It's not only straight-through. You get brightness either side as well, and you might even have noticed alternating brighter and dimmer bands as you move away from the straight-through position. These are all real. Try it out with a small enough slit, and you can see these alternating bands. Without a laboratory, try touching your first finger and thumb, in front of a bright light.
Reasoning about photons exploring space goes on giving. Another prediction using the same ideas.