Just to add some more thoughts about it...
i have just watched the video from kerrash, and i had one thought.
he said that the Thargoids control much more space than we do.
so...i havnt played the past few games, but i was pretty much interested, so i read some stuff about Thargoids.
There i noticed one thing that might be a clue.
The Thargoids have insane conrol about the Warp/Witch/Hyper - space. They were able to send commanders from deep space into their home systems, they were able to pull them out or redirect them.
And now...the Thargoids are gone.
So what if that "huge amount of space" refers to said Witchspace and not a real space? What if they are hiding in plain sight, inside the Witchspace and we travel through their space all the time without noticing?
I have read that Thargoids can survive in there, that they can basically enter a form of "stasis", while a human being would become insane if we would try that.
Also I have read that the Thargoids are not aggressive towards us, that we have started the war by killing them when they wanted to contact us.
They appeared out of nowhere and went back there, and they were never seen again since then.
They tried to communicate with us based on the lore, but we were to arrogant to accept it. We attacked them, and after several failed attempts to communicate with us - they vanished again.
What if they went away because we attacked them, showed them that we are not advanced enough to cooperate peacefully?
What if the UA/Sap8 is just a probe to see if we are ready now? If we are further developed and have more control about their Space?
So what if they want to contact us again if we show them that we have advanced, that we have learned more about their mastered space and that we have learned to cooperate peacefully (we cooperate through this topic, right?)
What if we need to carry that UA/Sap8 for a very very long distance in any direction (reference to the Diamondback) to trigger the "missjump" where they then contact us? one of them might be the "guide", and the other one allows us to "communicate"?
So the UA might be a communication device since it sends out signals in Morse that we can understand,
while Sap 8 was mentioned as a "map" -> A map is a guide, what if it is not meant to show us a physical starsystem where we find them? What if it is a guide that guides us through witchspace towards them, and all we need to do is stay in there as long as possible for it to activate (insanely long jump)?