Neutral ships leaving their system

[Baal]

I'm in the process of playing my first simultaneous game, so as to get used to it for multi-play. I've met 2 neutral empires so far and there is only 1 system between their 2 systems. I had a small skirmish between one of the Neutrals at an unexplored warp point. I destroyed his ship and got through the warp point with severe damage.

To my surprise the other Neutral which I had met earlier had ships in this empty system of asteroids. I thought what a coinky dink, that ship moving anomaly must have just moved them to the next system over. Many turns passed, the ships dissappeared, I thought they blew up.

A few turns later the ships show up again. Not only that but ships from the Neutral empire I had attacked were also now in the system.

Here's my dilema, Why are they popping out of their systems? And, when they pop out why don't they move around (They stay in the same spot for many turns).

Has anyone seen this before? Is it a bug?