You and Optimus recently rescued an autobot from an escape pod. You both brought her back. Arcee seems to know her.

Arcee: "She's one of my sisters. Elita."

Elita: "Yes, as my sister said. My name is Elita-1. But you may call me Elita."

Optimus: "Pleasure to meet you, Elita." Optimus greeted her.