I met a man, a Columbian, in Trujillo a couple years ago. He rented a room from a friend of mine. He was always out with his detector. He eventually found something like a gold cup, which he sold in a hurry for $2,000. He later learned that it was worth 10 times that amount.
There is a store that sells metal detecting equipment in Miraflores on Called Grimaldo del Solar (between Diez Canseco and Schell), so there must be enough people with this hobby to keep at least one store open.