The close reason indicates that questions here are expected to relate to scientific skepticism. Yes, these reasons are quite canned, but that makes no sense; the only reason I can think of to close this is that it belongs on Physics instead. If stuff like this is off-topic here, well and good, but I'd like to understand why.
Sorry about that, Fabian should have left a comment explaining to you why your question was closed. We discussed it amongst ourselves, and we decided we had to close it in the name of consistency even though it lead to an amazing answer.
The claim is just not notable enough. We have to do this, otherwise everything can be shoehorned into Skeptics questions for as long as it is worded as "a friend told me that..." and that is bad.
If you do find a place claiming that metal detectors can detect gold, feel free to edit your answer and flag it. I will gladly reopen it. The answer is pure gold.