Not sure if anyone else is aware but each terminal in Halo Anniversary ends with a code made up from 5 symbols.

if you fire up Halo waypoint and press X you'll be taken to a screen with some coloured circles. they correspond with X,B,A,Y. press what you see onscreen left to right and you'll be taken to another screen. Here you can enter the codes from the terminals to get some bonuses.

There is also an 11th Terminal that is unlocked after you input all 10 codes - I've not done that yet. You must have found the terminals in-game to input their codes apparently.