The posts at the top of page 9 of the IC thread covers where the party handed over the mysterious device. It was given to General Niall, who handed it to Major Yalor with orders to take it to Captain Bess to be studied, but this was also at the start of the Imperial assault, so it's exceedingly doubtful that Bess was able to learn much, if anything. Assuming that Yalor delivered the device to Bess. The next time you saw Major Yalor was in the Command Center just before you took Setenna to safety and General Niall was killed shortly after.
One of these NPCs is a traitor:
1. Commander Qurno (Duro)
2. Major Yalor (Sullustan)
3. Captain Harl Bess (human)
4. Lieutenant Var Narek (human)
5. Urel Haydon
It's unlikely to be Bess, since being a highly skilled tech he should have been easily capable of removing the device that was stuck in the main hangar. Common sense points to someone with minimal tech skills. It's unlikely to be Haydon, as any decent spy (even a double agent) shouldn't be quite so sloppy. You haven't discussed your concerns with Urel, yet, of course, but the traitor isn't particularly skilled as an agent (which is what Haydon would tell you). Certainly not a pro like Haydon.
That, sort of, narrows the list to Qurno, Yalor, and Narek. Assuming that Yalor survived (you haven't learned that yet), and he delivered the device to Bess (also not yet known), you still can't rule him out. You also need to know if the device was still functional when delivered to Bess (assuming it was delivered). Yalor, if he is the traitor, might have tampered with it so that Bess couldn't learn anything more. That would be foolish, as then he'd have to explain why it was working when it was given to him and not working when handed to Bess. Still, it's yet one more thing that you need to investigate.
As GM, I shouldn't be mentioning all of these points, but I'm giving the characters credit for being smart and being able to connect dots. Joran has a genius-level IQ (somewhere in the 150s). Sandra and V'Jax are brighter than average (120-125).
Narek heads your list of suspicious people. But you can't rule out Qurno or Yalor. They, too, have means and opportunity (although they're much "higher-profile" than Narek, so it'd be much harder for one of them to do something suspicious without anyone else taking note). Narek is human, and thus most likely to be viewed by Imperials without disdain, and (aside from that breathless moment in the Command Center) has had much more freedom to wander around without anyone paying attention to his comings and goings. Your problem, besides a lack of hard evidence connecting one of the three to recent events, is that you don't have any motive as to why one of them is a traitor. You do not know the backstories (at least not yet) behind any of the NPCs. (They're in the book, but it's not the sort of stuff a decent GM just hands out to players. Personal histories have to be discovered. Just as you would if you met someone at Mickey D's and started chatting with them. It would take a while before you learned anything private about them.) You don't know what planets they're from, nor why any of them joined the Rebellion. Talking to each one is a start. So might be enlisting Haydon to lend a hand. After all, Haydon was a professional spook.