Maybe Echo found it before he rejoined the bad batch?
Really sad prediction time:
This makes me sad but this has been something I have worried about since Rex first showed up in the bad batch back in season 1.
Right now, we see him determined to fight. He is eager and ready to help his brothers and other groups escape from the empire or ready to help those who want to fight the empire get the recourses they need. However, when we see him in Rebels, he has no desire to fight. He’s tired, discouraged, and doesn’t believe that fighting the empire is worth it. This means something big and bad has to happen between where Rex is in the Bad Batch to where we see him in rebels
AND IM SCARD. BECAUSE ECHO. JUST. WENT. WITH HIM. AND GUESS WHO WE DONT SEE IN REBELS