The fivers featured art by specialist micro-engraver Graham Short, whose previous work, a portrait of the Queen on a pinhead, sold for £100,000.
But what fate did all the notes meet after they were first used to make purchases in December 2016?
Act of charity
One of the four fivers is to go up for auction to raise funds for Children in Need.
The note spent in Northern Ireland was returned to the gallery in south-east Scotland with a small, hand-written note reading: “£5 note enclosed, I don’t need it at my time of life. Please use it to help young people, kindest regards J…”.
The Scottish fiver was found in a card sent in Scotland last Christmas.
Mr Huggins-Haig said that it was not clear whether the sender had known the value of the gift they had sent their loved one.
The recipient told the gallery they had only checked the note “on the off-chance” it was one of the valuable ones.
They added that they wished to remain anonymous.
The first note to be discovered was the one used to pay for a sausage and egg sandwich in a cafe in Blackwood, Caerphilly county.
Mr Short said he had selected the town as it is where his mother was born in 1909.
However, the finder did reveal she intended to give it to her granddaughter as an investment for when she grew up.
One of the special £5s remains in circulation 12 months after it was used.
“The last note, the one spent in Melton Mowbray, is still out there, somewhere in circulation,” confirmed a spokesman for the gallery.
Anyone who finds the remaining missing note – with serial number AM32 885554 – has been advised to contact the gallery.