Unfortunately there is no evidence to support that Sakura has learned Slug summon. So for now I am going to infer that she has not learned it until proven otherwise.
It would also be more surprising that no one noticed the puff of smoke and the fact that she is gone?
Why is there a need for you to go all evidence or proof? I just listed that idea as one of the hundreds and thousands of possibilities out there. No need to try to act like L to just a mere daydream
And btw, just because there wasn't a panel of Sakura learning the slug summon it's still possible that she has it. It might appear suddenly, like how Sasuke summoned the hawk and the snake. Again, I'm just talking about possibilities.
The same goes for the lack of puff of smoke. It might just be shown later that she has indeed reverse summoned, without showing the process. Naruto might ask towards Kakashi, "Where did Sakura go?" and Kakashi may reply, "To her master's place" and that may be all the confirmation we need. Again just one of the hundreds of possibilities out there.
Edited by ramenanmitsu, 16 January 2013 - 02:23 PM.