How did Commander Erwin know that the spy/traitor(s) would be within the 104th training squad?


During the female titan arc, how did Erwin specifically know that the traitor
would be from the 104th training squad?




From [a well established Tumblr blog post](http://pop-culture-theories-
fan.tumblr.com/post/133481651378/how-did-erwin-know-about-annie):

> After the death of Sawney and Bean, Erwin then was able to narrow down the
> list of suspects, as only a soldier was able to kill the titans with 3DMG.
> Because of this, he went around asking cryptic questions to try to weed out
> liars and traitors, making people second guess themselves.
>
> Then comes the night of the ceremony where the cadets join the Survey Corp.
> That night, Erwin instructed Levi to take Eren out on a ride to recon the
> area. Why exactly would he do this? It's clear it wasn't just a normal
> exercise, as Levi was rushing Eren and seemed frustrated and stressed. This
> was Erwin's way of narrowing down the suspects. By having Eren leave and the
> ceremony on the same night, he could deduct that if the enemy did _not_
> attack Eren or at least show up to the castle to kidnap him when it would
> have been strategically best to do so, then it _must_ mean that they were
> otherwise preoccupied somewhere else. Where else? The ceremony. If someone
> was missing from the ceremony, it would be suspicious and draw attention to
> them. So this narrowed down the suspects to the 104th Trainee Corp.

