Grand Admiral Thrawn was visibly shaken in Ahsoka when he was initially informed of Ahsoka Tano‘s presence, and that’s everything to do with the secrets he knew. Because Ahsoka wasn’t always on the front lines of the Rebel Alliance‘s fight against the Empire, Thrawn and Ahsoka never crossed paths during the Galactic Civil War. Despite this, Ahsoka knew enough about Thrawn to render him a threat, which is why she endeavored to chase down the rumors of his return in the years after the war ended, as seen in both The Mandalorian and Ahsoka.

https://screenrant.com/star-wars-grand-admiral-thrawn-ahsoka-tano-connection-factoid/