They moved, most likely, out of the city.

Creatures are always moving, so they probably just moved elsewhere outside your sensor range. I don't believe they can cross-terrain.

By that I meant they "technically" moved out of the city, because I was under the impression they can simply disappear if you enter a ship or if they're not in sensor range of anything.

I was somewhat foolishly operating off the assumption he had enough sensors to see the whole city or had done an extensive search already.

Admittedly I could be wrong on both fronts.

I think it was the original intention that they disappear when you enter an entity, but since people were saying they could enter their ship, fly after them and then continue killing them, I would say that they do not disappear if you merely enter an entity. Perhaps if you leave by ascending/cross-terrain yourself though.

Thanks for the answers. I actually found some more creatures and the original pack that I couldn't find earlier. Turns out they just keep going around in the city until they are killed.

I am chasing down a pack of Gorgs right now. I shoot, wait the fire delay, get back in my speeder, fly to them, and shoot again without them "disappearing".

As far as I can say they only dissapear when you leave the city square and there is no other PC.

For future reference (specifically to the OP), please don't alter the original topic nor remove the original question as all threads are archived and may potentially help others in similar situations. Altering the topic makes it exceptionally difficult to find the relevant thread after all.