A follow-up poster replied "confirmation bias". I have to say "so what?"
To function as a bomb detector, all it has to do go bleep when it detects some kind of thing, used in some kind of bomb. Chemical compounds, object density, ferrous metal content, anything.
To be useful as a bomb detector, it doesn't even have to do that - it just has to help reinforce safe handling procedures for "unknown objects".
False positives don't matter - if you have a device that, one time, keeps one operator from doing something that causes a live bomb from going off and killing him, then you've got a win. False negatives do matter, if the person operating the device doesn't recognize the possibility of a false negative and simply relies on the device to say yes or no. Confirmation bias? Someone who finds bombs on a regular basis is likely to recognize signs, consciously or unconsciously. To that end, a dowsing rod would be as useful.
So even if the devices were PURE snake oil, they'd still be useful as a mental prop.