核心畫面
Picture a machine with one narrow talent: it cannot see, but it knows when metal is near, and it says so out loud. Sand, soil, a coat pocket, a bag — none of that stops it. A metal detector can be thought of as that single beep standing between hidden metal and a person who wants to know. Everything else about the word is just a question of shape: the machine moves over the ground, or you move through the machine.
metal detector 可以想成一台只會一件事的機器：它看不見，但金屬一靠近它就知道，而且會叫出來。沙子、泥土…更多收起
metal detector 可以想成一台只會一件事的機器：它看不見，但金屬一靠近它就知道，而且會叫出來。沙子、泥土、口袋、背包，都擋不住它。這個字的核心畫面，可以想成「藏起來的金屬」跟「想知道的人」之間那一聲嗶。剩下的差別只是形狀：不是機器在地上移動，就是人從機器中間走過去。From the word’s history
The supplied Wiktionary entry gives the formation as metal + detector — a device that detects metal, which is exactly what both uses do.
手上的 Wiktionary 資料寫的是 metal + detector：把金屬偵測出來的裝置。兩個用法做的都是同一件事，字面幾乎沒有轉彎。
Wiktionary entry for metal detector, kaikki extract source_ref kaikki:3f6f69a190036ae4:959456171:11329 — etymology_text: “metal + detector”.