Setting off a metal detector when you aren't carrying obvious items like keys or coins is usually due to "hidden" metallic components in your clothing or medical implants within your body. Common culprits include the metal shanks in high-end boots or dress shoes, which provide structural support to the arch. . Underwire bras, heavy zippers, and metallic buttons or "studs" on denim can also trigger the sensors if they are sensitive enough. On a biological level, if you have had surgery involving orthopedic hardware—such as titanium hip or knee replacements, plates, or screws for broken bones—these will almost certainly set off a standard walk-through metal detector (WTMD). While modern titanium is often less "magnetic" than older steel implants, many high-sensitivity machines at airports or government buildings will still flag them. Other less common causes include certain types of body jewelry, foil-wrapped medications in your pocket, or even heavy metallic embroidery on high-fashion clothing. If you have an implant, it is helpful to inform the security officer beforehand; you may be directed to a millimeter-wave scanner (the "body scanner") which can visually differentiate an internal implant from a concealed weapon.