From Halopedia, the Halo wiki
Flag Returned is a medal awarded in multiplayer for returning your flag to your base in Halo 2 and selected games in Halo: The Master Chief Collection (Halo: Combat Evolved, Halo 2, and Halo 2: Anniversary multiplayer). It is available only in Multiplayer CTF gametypes where touch return is enabled.
In Halo 2, it is represented by a green circular medal with a flag. It is not observed during gameplay and can only be viewed in the post game lobby.
The medal has a Bungie difficulty rating of 1 (out of 10).