It's very easy to spot as the spot where it's slightly buried glows a little like treasure do.

In the chest they will find... well, bones, yes. But they're Golden Bones. And there's a note that says "There magical bones will aid you once if you get in trouble".

They really are magical, but you need to carry them with you to actually work. If a character's life is in danger these bones will become a golden skeleton and save them from harm (you can be creative how it happens). After that they will disappear into golden dust.