: The message is annonymous, but judging from its content it must be one of the
: Marathon AIs. There are only 3 AIs, and since Leela was ruled out in the
: message and I doubt it's Durandal, I'm assuming this is the
: thought-to-be-dead Tycho speaking.
No, I'm pretty sure that it's Durandal. In Marathon 2, on the level "The slings and arrows of outrageous fortunes", it mentions that Durandal brought the Pfhor to Tau ceti, not Tycho.
: Does anyone know where this terminal is on the map?
Yes, first go to the large room near the beginning of the level (the one that you can see through a window at the very start of the level). Go through the door that leads to the main part of the level, turn right and go up the stairs. At the top of the stairs there is a doorway in the wall. Go into the doorway and go into the room at the left. This room has two open spaces on each wall. Go into the one that is right next to the one with the shield recharger. That is a secret door. When you go through the door, turn left and press the action key at the wall. It will open another secret door in front of you. The terminal is in there. If my explanation wasn't specific enough then you'll probably be able to find the secret with the link below.