They didn't keep the mission a secret because it was dangerous, they kept it secret because they didn't want everyone to panic. Especially about something that might not even happen. Just like how they kept Naruto and Sasuke's fight a secret.@bold 1? how is that fanfic? Is it not enough that Sasuke asked to keep the mission top secret and the 5 kages agreed to not tell anyone because it was dangerous. Context clues is a basic concept in reading.
@bold 2. He obviously found traces or he wouldn't be in the dimension.
Sasuke can be followed without him knowing doesn't mean he is stupid. It would just mean it was beyond his ability to notice. Shouldn't be uncommon when it's someone stronger than KAguya. Only Sasuke can follow the traces he said that himself.
Btw what else did I ignore? Should I really pick at every little detail when the movie will probably clear all of our questions because it is the continuation after all. Thread is about what Gaiden has taught us.
And He hasn't found any traces of the new enemies, that was just a guess. He's looking for traces of Kaguya.