There are many places they could have been hidden and never found.
They could have a little festival every year to look for it ...
I mean, that spot in your neighbors yard that looks freshly dug would probably be a good place to look. (They were occupying the whole country)
So, not going out in the middle of the night in ninja swit; but chearful gardening in the Church's front yard instead.
You can even put a heavy, thorny bush on top, so people will be too lazy to undig it by accident.
I'd probably have tried to melt them, though.