Saw a guy demonstrating one like that at the Talbot footy ground caravan park when I was staying there a few years ago.
He dialled up the code for gold on the digital keypad (apparently you could dial up other metals or gems) and demonstrated in front of a group of prospectors of various belief levels.
He walked past a number of us wearing significant nugget jewellery around their necks before declaring there was no gold in the vicinity. So apparently it only works for gold in the ground not at chest level.
The believers still believed despite their own eyes.