According to Dave Filoni, he is referring to KOTOR, which has events that are now canon. BUT he is also referring to the "lost episodes of the clone wars."
Before Disney cancelled the clone wars, there was going to be an episode where Ahsoka, Rex, Bo-Katan, and Death Watch team up with the Clone Army to overthrow Maul's rule on Mandalore.
The episode would end with Ahsoka defeating Maul, before he escapes, and Mandalore joining the republic. While this was happening, Anakin and Obi-Wan were dueling each other on Mustafar. Order 66 happened, and Ahsoka and Rex had to flee from their own soldiers. Mandalore got handed over to the empire, and the war was lost.
As mentioned in the Rebels, Rex and Ahsoka last met since then. Dave Filoni said that the very last scene of the Clone Wars would have Ahsoka trying to reach out through the force to see if Anakin was still alive, only to hear the heavy breathing of Darth Vader.
Rebels Recon goes further into the Sabine/Kanan episode.
"So we fix our eyes not on what is seen, but on what is unseen, since what is seen is temporary, but what is unseen is eternal."
