While there are devices and systems that can detect forbidden items hidden on a person's body, in or under clothes, some traditional metal detectors frequently miss small contraband, like lighters, especially when concealed, due to the small metal content.
As explained by Minelab, metal detectors work by transmitting an electromagnetic field from the search coil into the ground. Any metal objects (targets) within the electromagnetic field will become energised and retransmit an electromagnetic field of their own. The detector’s search coil receives the retransmitted field and alerts the user by producing a target response. Minelab metal detectors are capable of discriminating between different target types and can be set to ignore unwanted targets.
Here are The Best Metal Detectors of 2017 compiled by Top Ten Reviews:
1. Garrett Ace 250
2. Bounty Hunter
3. Fisher F4
4. Teknetics Delta 4000
5. Teknetics EurotekPro DD
6. White's Coinmaster Pro
7. Fisher F22
8. Minelab X-Terra 305
9. Tesoro Cibola
10. Barska Winbest Elite 200
You can learn more on how metal detectors work and learn about the different types of metal detectors at HowStuffWorks.