https://en.wikipedia.org/wiki/Rosetta_Stone

Currently in the British Museum

If it's transferred to some other owner, that's fine as long as its location or ownership is public. It doesn't have to be displayed, but it must have a known owner. If the custody chain is lost and it's not visible anywhere, that would count as lost.

If the situation is uncertain, I may delay resolution until there is clarity. If there is no clarity after 6 months, then I may resolve the claim as YES it is lost.