I found this out myself, believe it or not. I call it "motorcycle glitching".
This glitch, allows you to go through walls, get under the level, and around the level, while being very hard to be seen.
Okay, this glitch could take some practice to get used to, first get a motorcycle. We'll use Eder Dam for an example, as I know every glitch in that map with a motorcycle... First, go to the American spawn point. On the left side of the house, their'll be a stone wall with a hill. Drive up next to it, with the front of the cycle facing the stone wall. Make sure, that the side you get out of, is almost wedged into the wall. Now get out, you'll be behind the wall, no one can see you, except when yu go up the hill. Be careful, you can fall through the floor. That's one glitch spot. There are another 10 or so in Eder Dam. Good luck finding them.

Choose chapter select and hold down select and press Right, Right, Left, Left, square, square.

Unlock All Levels, And Bonus Features- Go to the Chapter Select Menu, and hold down the Select button, then using the D-Pad, hit: Right, Right, Left, Left, Square, Square. (DO THIS QUICKLY!)

Happy gaming =p

(GOT IT FIRST DAY OUT BABY WOOT!)