I have been thinking about this for ages.
Surely Echo figures out Order 66 before he even graduates training because he’s a nerd and realised there is a missing Order from the Reg Manuals and either researches about it or asks one of his superiors and it triggers them.
I’m not a big enough Star Wars fan to know if a) the clones knew the order/ it was written (just not addressed), but just the vocalisation of it triggered the chips , or b) there was clearly just a missing order
and no one read or cared about regulation manuals to notice it there/ or missing
Either way, I find it hard to believe that Echo wouldn’t have noticed something missing or asked questions about this strange order that called for killing ALL Jedi. This boy is literally named after his obsession with relaying orders, and is made fun of for casually reading reg manuals. If anyone noticed, it would have been him.
Alternatively, Kix is still around when Echo is rescued, and could have mentioned something about Fives’s theory to him, and Echo puts it together because he has always been curious about this missing/ unusual command order. Perhaps he was less vocal about it when he was in training / 501st, and the Batch promotes him to think outside of the box.












