Now that people are creating insect-like drones, I’ve been thinking about how to make them invisible -to one observer, anyway.
One way is as illustrated. A very small, manoeuvrable drone (d) is equipped with a camera which informs it of exactly where the observer is looking (x).
Irrespective of whichever ‘x’ one is looking at, the drone forms one image on the blindspot of one eye (b) and, by flying very rapidly from position to position, it forms a stable image (s) on the back of the other eye (which then fades out).
This allows the drone, given suitably fast electronics and flight dynamics, to stay unseen.