It's a trend that's spread across the Atlantic from California: hiding envelopes filled with free money in secret locations and sending people out on a treasure hunt with clues posted on Twitter.
After a HiddenCashUK Twitter account sprung to life, there's now a London version inspired by the Californian movement. It already has a 2,700-strong following after it reportedly left its first envelope in a bush near White City Westfield.
The lucky winner of the first envelope was @Tatzikatzi who said they lived nearby and found the money in a bush.
The account holders of LDNHiddenCash told London Live that they are hoping to do the next package on Tuesday, "probably in the South West area of London."
London Live also spoke exclusively to the 25-year-old businessman behind the HiddenCashUK account, which has gained a following of over 60,000 in just a few days. After dropping envelopes filled with £50 in Leeds, Manchester and Sheffield, it send Londoners out on a scavenger hunt, also on Sunday. The money had been hidden in Camden Lock.
The US and UK benefactors behind the Hidden Cash accounts want to remain anonymous, and say that it's a social experiment.
The man who started LDNHiddenCash says his quiz will last at least for the month of June.
"We may carry it on further depending on how people take to it," the account holder told us via email.
"It's mainly an act of generosity and a bit of fun to see how London's people respond to it at a time where anything free, big or small is greatly appreciated.
"We're also interested in what they'll use the money for, as in San Francisco and other areas the movement has reached people who were using it to spread their own generosity - buying pizza for co-workers or donating to a chosen charity etc."
Some have complained that the £20 in the envelope wasn't enough. But the London benefactor, who claims he's not rich and describes himself as a male in his early 20's, says that his team have overall had a good response from the first drop.
"People seemed to love the idea and the mystery of it."
"It's been said by a few that we have an agenda, to gain fame, publicity, that it's for marketing purposes, or purely to gain real Twitter followers, none of which are true," he says. "It's all for fun."