In my first game with actual spacefaring NPRs, I'm seeing some AI behavior that I'm having trouble understanding.
We made first contact when they came into Sol, and every few months or so they'll send a ship in to visit Earth. They'll just sit there (either next to my Luna colony or 10,000 km off Earth) for about half a day of 60-second increments, without either of us being the first to shoot, and then vanish. Based on my passive and active sensors, the only way I can think of that they could have left without me seeing them leave is if they managed to make the normally-several-day journey to and through a JP within 60 seconds, so the only thing I can think of that they're doing in game-mechanics terms is that they're deleting their ship.
Is there anything else they might be doing, and if they are sending in ships and deleting them, is that a bug?