If you are an android user then there is an app called metal detector developed by smart tools, which let you to use your phone as a metal detector. It uses magnetic sensor to which measures magnetic field values that is built into the phone. It can detect from smaller metal objects to electrical wires in the wall.
You can turn ON or OFF the alarm sound and if the app shows you an inaccurate reading then there is an option to caliber your device. However the accuracy of detecting the objects in your surrounding depends totally on the sensors of your phone. Also, it can't detect some stuff like aluminum metals, coins, etc.
Color indication: If it's showing...
- Green, that means, magnetic field level in nature is within tolerance.
- Red, that means, magnetic field level is too high, indicating that there is a metal object nearby.
Check this video to see how it works