From the fact that Levi was in Bavel, Rav derived that R' Efes passed away.
He derived it like this:
After Rebbi passed away, R' Efes took over Rebbi's position. Rav Chanina was equal (or greater) than R' Efes in Torah learning (but not age, which is why Rav Chanina had R' Efes take over), so he learned outside. He needed someone to learn with, so he learned with Levi. When R' Efes passed away, R' Chanina took over his original position, so Levi (who was equal or greater than R' Chanina) had to go somewhere else to learn.
So we see from hear that Levi ≥ R' Chanina ≥ R' Efes.
However, the Gemara asks how did Rav know that R' Efes passed away? Maybe R' Chanina passed away. The Gemara answers that Levi would go to R' Efes' shiur. In other words R' Efes > Levi.
How does this work out?