My first post in several years to tell you all that the alert carry in Halo 2 is gone in matchmaking for whatever reason, you can still do it in customs though. Fix it 343 so I can RP again.
You have to hold the grenade button to alert carry in H2. It still shows as if you're not alert carrying in first person, but you're definitely with guns down in third person. In H2A, you need to hold the buttons assigned to the following moves: Zoom, Jump, Action, D-pad down.