There's a mystery at the heart of the UK economy...
it's the exact whereabouts of bank notes worth around 50 billion pounds.
That's about 65 billion dollars.
Experts were baffled this week after the National Audit Office revealed the number of notes in circulation has soared.
Despite cash declining in day-to-day transactions, the number of notes in circulation reached a record high of 4.4 billion this year.
In monetary value that's 76.5 billion pounds, or 99 billion dollars.
That's about three times the amount twenty years ago.
But the National Audit Office said the Bank of England in 2018 estimated only about 20%–24% of the value of notes in circulation were being used or held for cash transactions.
While UK households were holding on to a further 5% as savings.
Britain's independent public spending watchdog said in a report, "Little is known about the remainder, worth approximately 50 billion pounds".
It added that possible explanations include "holdings overseas for transactions, holdings in the UK of unreported domestic savings, or for use in the shadow economy."
The Audit Office said the Bank of England and other institutions should improve their understanding of why demand for cash has increased.
It said this might help inform wider policy on issues including tax evasion.
And maybe help track down the missing notes.