As the preview for Star Wars Rebels season 2 indicates, Ahsoka is sending the crew out to find Commander Rex and his fellow surviving Clone Troopers. But Anakin’s former padawan isn’t sharing the specifics of their quarry with Kanan; “You must trust him,” are the only instructions Ahsoka offers.
The reason for this is pretty clear — the Clone Troopers were responsible for carrying out the ruthless directive of Order 66, which wiped out the Jedi Order. Kanan, a padawan at the time, lost his master in the attacks and barely survived being brutally hunted by the Clones he once called comrades. Clearly, Kanan will have slightly complicated feelings when he meets Rex, and it’s hard to say whether or not Ahsoka is correct in keeping him in the dark.