They may have fallen out of someone's pouch as they were detecting(to make room for more coins and jewellery), maybe someone has an even betterer detector that we've never ever heard of before, some crazy inventor maybe that will never share his secrets(just a theory) and would rather take them to the grave,... it's probably got some worm type retrieval method(like a doctors instrument) that pushes its way through the sand to grasp the target and bring it to the surface(leaving only a small hillock of spaghetti like sand if anything at all, as a clue to him having even been there in the first place),... much akin to what happens in a colonoscopy to get rid of a blockage(large coin).Any time you hit a beach with no targets at all(except pulltabs/ringpulls) you could bet your bottom dollar(the colonoscopy coin) that old crazy has been there already and ripped anything out down to about six feet deep. If your finding anything at all it's probably that you've scared him off,... and he was the guy leaving the beach just as you got there(couldn't even tell he had a detector with him, could you ?).