The first was found in change from a café in south Wales earlier this month.
Two more special £5 notes, spent in England and Northern Ireland, remain in general circulation.
The two banknotes bear the serial numbers AM32 885552 and AM32 885554.
Anyone finding one of the notes has been advised to contact the Tony Huggins-Haig Gallery in Kelso, in the Scottish Borders, which launched the project.
Mr Huggins-Haig told BBC WM the recipient of the latest note, who had checked the note "on the off chance", wished to remain anonymous.
"When somebody opened their Christmas card from a loved one, it was contained in that Christmas card," he said.
"The person who put it in didn`t necessarily know what they were doing.
"That`s two down and there`s still two out there. Keep checking your change."
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