The old one-krona, two-kronor and five-kronor coins, as well as the old 100 kronor and 500 kronor banknotes, ceased to be legal currency on June 30th this year, after being replaced by new designs.

It followed the old 20, 50 and 1,000 kronor bills also being replaced by new banknotes a year ago.

In an effort to get people to hand in their old cash, the Riksbank, Sweden's central bank, launched a campaign last year featuring “Wanted” posters on billboards, in newspapers and in digital channels.

But according to its latest estimate, 5.9 billion kronor's worth of old notes and 1.8 billion kronor's worth of old coins are still missing.

Since October 2015, when the money changeover began, 90 percent of the old banknotes have been handed in, but only 35 percent of the coins, according to the Riksbank.

It is still possible to hand in the old one, two and five kronor coins to a bank until August 31st 2017. The deadline for handing in the 100 and 500 bills is June 30th 2018.

Even if you miss those deadlines, you can still send the notes to the Riksbank, but it will then cost 100 kronor to get them deposited.

Exactly what Swedes are doing with all the missing cash is not clear, but there's a good chance that much of it is hiding in drawers in the famously cash-averse country.

Sweden is one of the countries that has come furthest towards becoming a cash-free society, with cash transactions accounting for just two percent of the value all payments.

Research commissioned last year by the credit firm company Visa suggests that Swedes are not only using payment cards more often than people in most other nations, they are also using them for smaller amounts of money.

According to Visa, the average card purchase in Sweden amounts to 301 kronor, while the European average is 459 kronor. People living in Sweden also use their cards more regularly than those living in all other countries except Finland, the study suggests.