The achievement is located in the 10th chapter called "Rust".
In the middle of chapter 10 you will come to the engine room. Here you find a car, a magnet and on the ground a control unit for the magnet.
Basically you have to do the following steps:
1) Turn on the magnet
2) let the magnet down into the car (a steel cube will attach)
3) drive the magnet and the cube to the left side over the wooden crate
4) turn off the magnet
5) turn on the magnet and get again the steel cube
6) turn away (other side) the magnet and collect your secret (it is number 4 in this chapter)