I think the only person who would have thought it is Pansy.

So in a way I'm glad she was the one who found the locket, despite what it did to her.

And there's no telling how long it would have taken the rest of us to find it, if she hadn't. Who'd think to go looking for a horcrux in a house-elf's nest? Of all the places...

How many more do you suppose there are?