The piece of jewellery was believed to have been found by staff more than 12 years ago at Castle Fraser in Aberdeenshire - the most elaborate Z-plan castle in Scotland.
When no-one came to claim it, staff placed the silvery pendant in a drawer and it was forgotten about - until last week when it was found again.
Now staff at the castle hope to harness the power of social media to return it to its rightful owner.
And thanks to the wonders of the modern world the search has gone global, with people sharing the appeal as far away as Canada.
Paula Swan, who manages the 15th century property for the National Trust of Scotland, said she would love to get the precious pendant back to whoever had lost it.
She said: “It could have been here longer than 12 years. That’s just from staff members that have been here for a while, they remember it dating back at least as far back as that.
“It’s been put in a drawer and just sort of forgotten about.
“Castle Fraser has been open to the public since the National Trust got Castle Fraser in 1976.
“We would assume this belonged to a visitor or a walker using one of the trails. We don’t know where it was found.
“The locket itself is quite bashed so it may have been found outside.
“It’s a mystery.”
Paula described the locket as bigger than a two pound coin, and oval in shape.
Inside are two distinctive photographs.
The photo on the left-hand-side of the locket is a black and white shot, showing a young boy and girl holding a baby.
The picture on the right-hand side shows an elderly man with grey hair holding a mug or tankard up to the camera.
Anyone with any information about the locket’s owner is asked to contact the National Trust for Scotland.