he comes back from a fierce battle and sees you, he doesn't pay attention to you and traces his path,he goes to Erwin and he seems to be talking to him about the battle

Erwin:Did it go well ?

Levi:We did everything we could to slow them down as much as possible.

Erwin:Alright

you spy on them talking and Levi notices you

Levi:Get out of there now,I've see you, why are you spying on us? Are you a traitor ?