NOTE: SPOILERS AHEAD, STOP NOW IF YOU HAVEN'T FINISHED THE GAME

Stadium City (3)

#1. In the area after the first fight with Red Sixer vehicles, there's a fire barrel behind a pillar. Shoot it and it'll break open the wall. Go inside and up the elevator to find the Company bike.
#2. In the tunnel after that same area, Rant will speak thru the radio. There will be a lone fire barrel on the right, roughly around the same time Rant starts talking. Shoot it and it'll break open the wall. Go inside to find a small area.
#3. After that same tunnel, on the right, there's a barrel (not on fire) once you exit. Shoot it and it'll break open the wall. Go inside and it will lead you to a small bridge that you can go across to access extra items.

Yahoos (5)

#1. When you start the game, turn around. Place your car at the front of the blocking semi's hood. Jump on your car and then on the semi to get a secret cutscene.
#2. After destroying the second accumulator, don't go into the hole that appears. Instead, turn around and head towards the right, which will lead to a bridge. If you cross the bridge, an enemy will appear on the other side, and there will be extra ammo inside. This small area is actually closed until that second accumulator is destroyed. If you go there before destroying it, then there will be a locked door blocking the way instead.
#3. After dropping into a hole at the second accumulator, there's a garage with a crack inside it straight ahead. Shoot the garage to reveal a secret car.
#4. After destroying the third accumulator, there's a secret pathway just behind where the accumulator stood, with machine gun ammo at the entrance. Follow the path to find Dean and have an alternate ending.
#5. In the next area with the multiple accumulators, as soon as you enter the area, if you goto the top-right corner, there's a glowing radioactive box. Blow it up, and it'll open up a hole in the ground to reveal a secret car.

Toxicorp (4)

#1. There are two ways to gain entry into the Templars' fortress. The first method is to not kill anyone upon approaching the entrance, and since you're in a Templars car, they will let you in. The second method is to kill everyone you see, and then on the wall to the left of the gate is an override switch, which will allow entry.
#2. When entering the main courtyard, there will be 3 ventilation fans on the left, next to the enterable building. If you blow open the third one (farthest from you), you can jump inside for a secret path. As you drop down, enemies will continue to drop in behind you. They usually have ammo for powerful weapons.
#3. Once in the underground section, there is a room where, if you enter it, an enemy will run to a control panel. If you allow him to reach the panel, a Dragoon will appear, and he has powerful ammo.
#4. In the control room where you have to push the button to activate the elevator, there are small black control panels that sort of resemble computers. If you destroy all of them, the gates will open up just outside the control room, thus giving you access to the catwalks and turret.

Red Six (1)

#1. When encountering the enemy on the bridge wanting to play chicken, simply reverse and get out of the car (to the right). There's a button on the wall that will re-close the gate, and he will easily kill himself.

Airport (2)

#1. After destroying the plane and exiting the turret, if you head down to the ground level, underneath the airport terminal, there is a button between 2 baggage claim belts. If you press it, it will deactivate the electric screens that previously blocked the baggage claim belts from above, thus accessing ammo and powerups.
#2. After getting the EMP and having to escape, once you reach the dirt mound at the end, exit your vehicle as soon as you're flying above the wall. You will drop on-foot over the wall, and will get a special radio transmission from Liddy.

Freakway (3)

#1. At the first area, fly all the way up to the top of the large fire stack. This will unlock an extra car in the second area.
#2. On the way to the second area, there will be 3 separate switches above that can be shot at. This will spawn an extra vehicle in the second area just before you enter it. Push the second vehicle with your car so that it doesn't get blocked out when the lasers come on.
#3. In the third area, if you go on-foot and reach the middle of the top, there is a switch that can be pressed. It will disable the orb that blocked the extra ammo and powerups in the middle.

Challenge (3)

#1. If you enter the HQ building on-foot, and press yourself against the speakers on the wall, it will open up a door leading to extra powerups and ammo.
#2. After Bork is dead and Liddy appears with his tank, an additional area inside the previously mentioned secret will be unlocked in the hallway.
#3. To get Liddy's tank, drive on the main road to the left, the direction Bork is initially facing. When you reach the orange cones, drive thru the middle of the cones on the right, then quickly drive thru the middle of the cones on the left. Don't pass up the left cone's front cone when making the movement. It's an " S " pattern. Continue driving down the road to find Liddy's tank on the left.

Sanctum (5)

#1. When you drive up the first ramp you encounter, there will be a small wall that you can use your car to jump over. This will give you access to a special vehicle. The vehicle can also be accessed by destroying the wall that blocks it in later in the level.
#2. At one point in the level, after opening a large door by hand, there will be a small area of stone ruins to the right of the road. Health pickups are on the ruins, but of course you have to exit your vehicle in order to reach them; however, it's technically a trap. If you get out on-foot near the ruins, as soon as you reach the top, an enemy will suddenly appear on-foot and he will run straight to your vehicle, and will steal it unless you kill him before he reaches it.
#3. Once you reach the point of the level where you have to leave your car and go on-foot into the subway, instead, continue running down the street into the minefield. Stay to the very far left. Once you reach the building, hug against the wall until about halfway down. Then, go in a diagonal direction towards the civilian standing in the middle of the road. Push up against him to get a secret message. Then, continue walking in a straight line down the road. There will be 2 turrets just around the corner, but a secret car and powerups will be near them, and you can use the ramp to jump into the next area. The minefield is buggy, because sometimes if you try to drive a car thru the minefield, it will either explode, or, if you survive, you'll get kicked out of the car and it'll vanish; thus, going on-foot is ideal.
#4. After entering the Templars' HQ, there's an area where you have to push up against a red burning stoke. In that area, are people crucified on the walls. Kill all of them (one is hard to see, above the elevator). This will open up a secret area off to the side.
#5. Before the boss fight with Archmagus, during the part where you have to retrieve the serum, if you entered the area via upstairs, and you decide to backtrack, a large Dragoon enemy will spawn at the steps (regardless if you're on the left or right side). They drop strong powerups once killed and they seem to be slightly glitched as they sometimes won't attack you at all.

Boom (2)

#1. At the bridge just past the first turret you encounter, drive off into the canyon under the bridge. You will encounter a cameo appearance by Slave Zero.
#2. Before you arrive at the extremely large bridge, if you instead drive way off to the left, there will be a secret road that will allow you to completely avoid the Red Sixers' camp.

Area 51 (3)

#1. At the first area, if you goto the crashed UFO and destroy the square panel, you will be able to access inside of it.
#2. During the boss fight with Golem, if you look behind the stack of computers, there will be extra ammo on the ground. Non-explosive weapons will not hurt him.
#3. While underground with the aliens, you will eventually stumble upon a dead body not yet fully decomposed. Although it's not mentioned in the game, it can be assumed that this is Dean's fate.

Barrage (1)

#1. Once you reach the area with turrets up top and a very small ramp leading downwards, there will be a large gray boulder that can be destroyed. It will open up a path to a secret area, and the same civilian from Sanctum will be there.

Showdown (2)

#1. At the start, turn around and go down the ramp. If you look up at the ceiling near the briefing room doors, to the right, there will be a square control panel that can be shot at with the sniper rifle. It will open up a small compartment in the wall nearby.
#2. After accessing the compartment, look in the rectangle hole on the wall to the right of the briefing room. Use your sniper rifle to zoom inside. Just around the corner, to the left, there will be a small black square that will look like a switch. Shoot it, and it will unlock the briefing room doors.

Other (2)

#1. While playing any level, if you press and hold G and O and D (press and hold G, then quickly O, then quickly D), and type in the code teambeyond , you will access a secret credits finale, a cool level by Beyond Games.
#2. Not exactly a secret, but, in the On-Foot Training level, as you may have noticed, if you pickup a weapon a message will appear saying how to use it. As you're shooting the fire barrels with the sniper rifle, you'll notice that rocket ammo will appear on a platform. It's a little tricky, but, if you run up the hill, then jump and land onto the platform with the rockets, you'll get a message on how to use them, too. Not sure why it was placed this way. It's also odd that there are 4 weapons missing from training (minigun, gauss rifle, EMP, and singleplayer's alien goo gun/multiplayer's flash grenades).