How did Darth Vader trace the rebel spies to Princess Leia?

Question

When Darth Vader's ship captured the Tantive IV at the start of A New Hope, he tells an officer,

I have traced the rebel spies to her.

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How did Vader trace the rebel spies to Princess Leia?

I am looking for anything in canon sources that mentions specific spies whom are in contact with Leia. And please say how Vader knows the spies are in contact with Leia.

Edit to add:

Vader watched the Tantive IV depart at the end of Rogue One, but he says he traced the rebel spies to her instead of saying, "I just saw this ship flee Scarif with the stolen plans." Does he know of her rebel leanings from other sources besides merely being on the ship?

Just as Rogue One corrected inconsistencies in the original Star Wars movie (e.g. - one person said stolen data tapes and another said the ship intercepted transmissions) and filled in details (e.g. - the opening crawl of the original movie said the Rebel Alliance just won its first major battle), I would like to know if any canon sources provide details about rebel spies and their connections to Leia.

Answer 1

Star Wars: Rebels, S2E12, A Princess on Lothal shows Leia going to Lothal with three ships, supposedly full of relief supplies for the suffering civilians of Lothal.

When Leia lands, a local Imperial officer takes steps to secure the ships, since Alderaanian ships seem to have a habit of showing up in Rebel fleets. This indicates that there is already suspicion in the Empire that Alderaan, and specifically Leia, is involved with the Rebellion. The officer uses gravity locks to keep the ships from taking off, and adds a detachment of Stormtroops and two AT-ATs for security. Nonetheless, the ships end up being stolen by the Rebels.

Those ships, by the way, were Hammerhead corvettes - the same design as the ship that destroyed two Star Destroyers at Scarif.

So, Vader knows that Leia is suspected of being part of the Rebellion. He knows that ships that were under her command, as part of a "Mercy mission", were taken by the Rebels and used in the Battle of Scarif. He knows that her ship was involved in the Battle of Scarif, and has taken the plans.

This is a decent circumstantial case under any circumstances. Vader even throws her earlier excuse in her face, by saying "You weren't on any mercy mission this time". It's pretty clear even from that line that she's been caught before, and got out of it by pretending to be on legitimate business, but that this time that's not going to fly.

Answer 2

By physically following the data stolen by the spies to Leia.

He literally watches as rebel soldiers run the data tape (stolen by said spies) containing the death star plans to the Tantive IV in Rogue One. At this stage he knows that spies have infiltrated the facility on Scarif and transmitted the data to the rebel ship where the Tantive was docked at the time. He is later seen in the Hanger bay of said ship watching the Tantive depart. See this video:

In the movie at least it is unclear if he knows exactly who the spies are, I'm not sure if this is clarified in other sources. The spies in question are of course the main cast of Rogue One, including Cassian Andor, Jyn Erso and Bodhi Rook.

Why Leia specifically? Well she is the most senior political figure on the Tantive, which places her in a position of responsibility for what just happened.