Soldato
I watched a video on YouTube recently of a cruise ship turning around mid ocean to take a look at a life raft they'd spotted. Just watching that made me realise how difficult it is to see anything small like that at sea even with the high visibility markings on it.
No wonder these small drones are able to go unnoticed for so long.
Indeed its why the Navy protocol for a man over is to have multiple eyes looking and if spotted so many have the sole responsibility to try to keep eyes on.
Now imagine the drone is black and its night.
Even if noisy the noise is often difficult to pin point.