Flag Taken
From Halopedia, the Halo wiki
Summary
Flag Taken Medal
http://www.yesalbum.com/v001/thepretenders22/flag_taken.jpg
The Flag Taken Medal is awarded for taking an opponent's flag from their base. It is available in all Multiplayer CTF gametypes and has a Bungie difficulty rating of 4 (out of 10). It is represented by a yellow triangle with a flag. This medal is not observed during gameplay and can only be viewed in the post game lobby.