In addition to recreational use, metal detectors serve a wide range of utilitarian functions. Mounted detectors usually use some variation of PI technology, while many of the basic handheld scanners are BFO-based.
Some nonrecreational applications for metal detectors are:
Building security - screen people entering a particular building, such as a school, office or prison
Event security - screen people entering a sporting event, concert or other large gathering of people
Item recovery - help someone search for a lost item, such as a piece of jewelry: underground metal detectors.
Archaeological exploration - find metallic items of historical significance
Geological research - detect the metallic composition of soil or rock formations