There may be one in an old coat pocket or hidden down the back of the sofa.
About 169 million round £1 coins have not been returned to the Royal Mint, nine months after they stopped being legal tender.
The round £1 coin was replaced by the 12-sided version in October last year, with millions of the old coins being melted down to create the new ones.
But, while you can not spend the old coin, you can still take it to your bank and deposit it into your account.
The new £1 coin was introduced on 28 March 2017 to help crack down on counterfeiting, with one in 30 of the old version estimated to be fake according to the Royal Mint.
About 138 million round £1 coins have been melted down to help create some of the 1.5 billion new ones at the Royal Mint, based in Llantrisant near Cardiff, according to figures obtained by BBC Wales.
The new coin is described by the Mint as the most "secure in the world" and has a string of anti-counterfeiting details, including a hologram, and micro-sized lettering inside both rims.
It also has material inside which can be detected when electronically scanned by coin-counting or payment machines.
A spokesman for the Mint said it had expected about 85% of the 1.7bn round £1 coins - or 1.4bn of them - to be returned during the transition period, based on the number of coins returned when the 50p coin changed in 1997-98.
"We do not expect that all round £1 coins in circulation at the time of the transition will be returned to the Mint," he said, adding the numbers returned were in line with expectations.
"Also, based on the returns of other demonetised coins we expect there to be some returns for a number of years to come as people find these."
The Royal Mint declined to say how much money smelting the round £1 coins saved in production costs of the new £1 coin, saying that disclosure of the information "could put us at a material disadvantage in any future negotiations with our overseas customers for the manufacture of their coinage".
According to the Royal Mint, the old round £1 coin can continue to be deposited into a customer's account, either business or personal, at most high-street banks.
These include RBS, NatWest, Ulster, HSBC, Barclays, Lloyds, Santander, Nationwide, Clydesdale, Yorkshire Bank, Halifax, Bank of Scotland and The Post Office.