Secrets are ingame collectable objects with a glowing "S" on it.
This is for example a Secret:

By collecting the secrets, cheats will be unlocked, which can be selected before you start a new campaign.

After the first contact with the aliens, you have to go left into the small dead end. On the left side there is a chain link fence with a hole in it. Just go through the hole to the end.

Once you're at the gates of the facility, this is just before the end of the level, you have to go through a gap left behind a red barrel. It's behind some boxes in the storage.

There are 2 Secrets in this Level. The first is to the beginning of the level. There is a hole in the fence. There is a hole and you're a person. You know what to do.
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Once you've eliminated a bunch of boxes that have you blocked the way, you have to go right. There is a door that is blocked by crates. Walk through it and right through the locked door back out. The Secret is in the back corner.

At the end of the level is a door. NOT GO THROUGH THIS DOOR!!!
The secret is right in the dark forest. Get it and then go through the door.

After you have repaired all transmitter housings, this door opens.

Before you go to the scientist who is located in the satellite building, you should go through a small gap between the dumpster and fence. There is the Secret
It's also afterwards, but the scientist is attacked on the way to the landing bay by aliens.

Just go straight up to a large closed gate. There are in the corner a stack of boxes. In this stack is the Secret. Preferably simply clear the way with grenades and get the Secret.

After you went at the first turning on the right, just straight ahead. After a large gate comes a fence. At the end of the fence is a passage. At this cliff lies the secret.

Once the paths split the first time, you have to go right. The secret is hiding on a cliff.

The secret lies in a shelf in the arena. It is ziehmlich close to the platform with the surviving soldiers.
If you do not go to the platform, you can have the secret get in all peace.

This switch is located at the minigun. Pressures in order to open him a door.
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This door will be opened. It is located a little bit back to the left. There is a slightly longer route, but with a few more items.

Again, there are two secrets.
After you pass the first building, there is a hole right in the chain link fence. You know the drill.
On the right side is the secret.
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After you pass the first building, proceed straight until you hit on a white barrier. one of the barriers fell down. Behind it is the second secret.

At the beginning of the level, located to the left of the building, a whole stack of boxes. Yes, at the very end is the secret. Used your grenades by the boxes to clear out of the way to get it.

Right at the beginning of the level there is a turning to the left. At the end of this column is the secret.

Nearly at the end of the level is a large forest, which you must traverse. At the beginning of this forest you have to go up to the cliffs go straight into the dark forest. It should be seen quickly.

Pretty much at the end of the level a crossing should come with three ways. One to the left into a dead end, one straight ahead and one to the right. At the end of the right path is the secret.

Where did you downloaded the backup is a door. In the room behind that door is the secret.

After the Mech is down you come to an entrance. There you can go behind the fence. After a longer way, you get to the last secret.

That's it.

If it should still give fault in the text or suggestions for improvement, then simply tell me about it.