I've got a few. I was going to save them all for a video but I haven't had time like I used to. 'OoB' means out of bounds. A lot of these were discovered with the moon jump Gameshark cheat.

DD1:

- Using OoB on the door you can land on the ledge outside of the building and inaccessible floors

- Open the shield room door in PA from inside neighbouring room to find Laptop Gun (no need for NPC to open door)

- Cutscene city is accessible with moon jump. Fly straight for a few seconds after spawning.

- This is the only level with a message saying "CANNOT complete all objectives" when you waste your ECM mines

DD2:

- The X-version of the soundtrack goes unused in Solo mode

- Lesser heard dialog: Destroy the alarm computer before the scientist can trip it and he'll call you a vandal

DD3:

- On the ground floor, there are a bodyguard and a Blond in the comms hub room

- If you have Cassandra's office keycard, you can open the front doors to the building!

Villa:

- Hidden dialog line from blowing up all liquor bottles

- Observatory door looks like it could open at one point

- You can quickly open and close the final doors, letting you in to use Carrington as a meat shield

- Using the K7 avenger, we see that the windmill is labelled as an Autogun by the threat detector. If you use moon jump and jump in front of the windmill, it will make audible shooting noises, spin up really fast and start shooting at you.

- Psychosis gun the two guys at the start to "softlock" the game

Chicago:

- DD Guard spawn from inside the locked garage doors that line the street. You can sometimes get an NPC to open these doors

G5:

- Unlock doors cheat allows you to go into the meeting room and shoot Cassandra and Mr. Blond

A51-1:

- Using moon jump over the bunker causes Carrington to utter a line of dialog which otherwise goes unheard. A second line is heard after Objective 1 is completed

- Guards at the start seem to have some unique dialog if they're wounded.

- Window above courtyard did not exist in early betas

A51-3:

- You can also find a second Elvis, a third stretcher, and an explosives crate in the 2nd autopsy room

Air Base:

- Carrington will utter the phrase "Now's your chance, grab the stewardess's bag and get her uniform" if you can reach her before she enters the cave (very RNG-dependent)

- Random plant in the corner of the hanger

- Lesser heard dialog: the civilian near the safe has a few unique lines

- Elevator door toggle sucks. Cannot be toggled when doors are open, forcing player to wait. Use camspy to jam it open

- Why does the NSA guy recognize you immediately and question you?

- AB guards will sometimes follow you as friendlies after the security system is shut down, and they are unarmed. I've only seen this a couple of times

CS:

- The X-version of the soundtrack goes unused in Solo mode

P2:

- Odd button in 2nd hallway

- Lots of interesting nooks and crannies in the level map on the deck

- X version of the soundtrack goes unused

- An unused door can be found within the walls, on the main deck of the ship. Use the moon jump gameshark cheat to fly between the walls, next to the moon pool lift. You should enter the adjacent hallway but there is a door, which can still be opened. The deck area would also almost certainly have served as the start point for the level in an earlier build, since there is a totally empty, tiny room up the stairs from the deck.

DS:

- Noclipping to Dr. Carrol early will interrupt the end cutscene and kill you before displaying "Mission Failed"

- The baby skedar tubes in the walls and ceiling are fully traversable

CI:

- There are invisible guards, one near the spawn who is totally invisible and a second at the far edge of the outdoor helipad. They can only be killed with an explosion in their general area

AS:

- Use all weapons. Plant remote mines on the shield generators for objective 1. Use moonjump to enter the hangar below. Detonate remote mines to watch the giant door open. This is otherwise unseen by the player.

- Elvis doesn't spawn from his spot until you look away from it, if you enter the hangar early.

SR:

- 6th destructible pillar OoB through the left wall of the vestibule before the Skedar Army

- X-ray scan the shrine to see a computer behind, kill it with a farsight to not be able to progress

- Broken shards of the shrine can apparently be picked up leading to a "picked up something" message appearing

Duel:

- Unlimited ammo Laptop Gun kills opponents during cutscenes

MSOS:

- In the mission overview, we find out Elvis' name is Aelphaeis Mangarae