DETROIT – Catherine Gulledge, a longtime Salvation Army bell ringer, works all day during the holiday season standing just inside the doorway at a Michigan grocery store wishing a "Merry Christmas" to people passing by her red kettle.

Many stop to offer some change or a few folded up bills.

Last week, Gulledge said, one person – likely the same mystery donor who has been doing this every year since 2013 – surreptitiously slipped in a Krugerrand, a South African gold coin valued at about $1,300.

That's a lot of money, Gulledge said, considering that an entire day of bell ringing usually nets $300 to $400. The gold coin, she said, was a little smaller than an Eisenhower dollar coin and wrapped, like a Christmas present, in a one-dollar bill.

"I didn't know I had gotten it until someone who heard about it on the radio called me," said Gulledge, a 64-year-old Roseville, Michigan, grandmother, who has a soft, almost shy voice. "A lot of people now are coming by saying they know who it is, but can't reveal it."

Across the nation, an increasing number of anonymous do-gooders are dropping valuable coins – often gold, South African Krugerrands – into Salvation Army donation kettles.

The charity doesn't know how – or why – the tradition started or what will happen to it as an increasing number of people adopt to a digital era, with money changing hands electronically.

December 2017:Mystery donor drops $200K in Salvation Army kettle

Devices like the Square, which makes it easy to take credit card payments via mobile phones, and services like Venmo, which allow users to transfer money from bank accounts with an app, are becoming more common.

There's also giving through the website and even by text.

But for now, the gold coins keep coming in.

Over the years, news accounts report Krugerrands – and other valuable coins – have been left in kettles in cities across America, including: Gettysburg, Pennsylvania; Reno, Nevada; Lafayette, Indiana; Naperville, Illinois; Waterloo, Iowa; Fort Collins, Colorado, and Daphne, Alabama.

The Salvation Army cashes in the valuables and uses the money for charity.

More gold coin donations

A worldwide charity, the Salvation Army was founded in 1865 in poverty-stricken east London by the Rev. William Booth and his wife, Catherine.

The organization's mission is to "preach the gospel of Jesus Christ" and "meet human needs." It serves the poor, destitute and hungry.

The charity's first red kettles appeared in 1891 in San Francisco, when Capt. Joseph McFee decided to try using something he'd seen back in England: A large pot on the docks into which charitable donations could be thrown.

The Salvation Army said it doesn't track how many gold coins it receives.

But the donations, which are documented in news reports, seem to be a trend.

“There was a time when we didn’t know what a gold coin looked like – and we’re thrilled with anything given at the kettle," said Lt. Col. Ward Matthews, the Salvation Army Secretary for National Community Relations and Development, in Alexandria, Virginia. "But for some years now we’ve received gold coins worth more than a thousand dollars each, which do so much good for families.”

December 2017:Anonymous donor drops gold coin worth $1,255 in Reno Salvation Army kettle

At some locations, like at the Kroger in St. Clair Shores, Michigan, gold coins have become an annual tradition. Since 2013, someone has annually left a Krugerrand there.

Who is the donor — or perhaps donors?

Every year, local news reports of the gold coin donation resurfaces speculation.

A diamond ring, gold teeth

It's unclear when the practice of dropping gold coins into kettles started nationwide.

Perhaps it is the anonymous and untraceable nature of the donation.

Or maybe because when one is found it leads to headlines and chatter. That, in turn, inspires more donations and keeps the gold-coin giving tradition alive.

In Gettysburg, where a couple anonymously dropped gold coins into kettles for years, the whodunit mystery finally came to an end when the couple, Dick and Ruth Unger, went public.

Dick Unger told NBC News in 2015 that he initially bought the coins as investments, but decided to give them to charity instead and got a rush out of seeing the mystery annually reported in the paper.

Eventually, he said, the secret became just too hard to keep.

One of the larger valuable coin donations appears to be in 2006. An article that year in the Bennington Banner reported that a rare coin – a 1908 Indian-head with a face value of $2.50 – ended up in a kettle in Barre, Vermont.

The coin, which was enclosed in a protective plastic case, was valued at $14,000.

And just days before Christmas in 2011, CNN reported that the Salvation Army had received a total of about 40 gold coins, among other valuable treasures. At the time, the TV network said that in 25 years the charity estimated it had received 400 gold coins.

December 2016:Salvation Army bell ringer given Spanish shipwreck coin

Krugerrands – which were named after former South African President Paul Kruger, who is depicted on one side of the coin, and rand, the South African unit of currency – seem to be the most popular gold coin found in the kettles.

They make up most of the gold coins minted. But, Krugerrands also have a controversial past.

Krugerrands made up an estimated 90 percent of the world's gold coin market in 1980, according to U.S. Gold Bureau, a Texas-based private investment firm. Millions of them have been brought into the United States as investment.

During the 1980s, Krugerrands were prohibited from being brought into the United States to pressure South Africa to end its policies of national racial discrimination.

South Africa ended apartheid in 1994 and there's no longer a ban on the coins.

In addition to Krugerrands, reports say Salvation Army kettle donations have included other valuables: An anonymous donor in Spokane, Washington, gave a diamond ring; someone from Fort Myers, Florida, offered gold teeth.

'I like to give back'

Gulledge – who first put on her red, bell-ringer apron in 2005 – bundles up in a hat, scarf and coat.

Gulledge said she enjoys her work because she "likes helping people."

She said she has no idea who put the coin in the kettle. So many people pass her all day she can't remember them all. But, she said, she is convinced the same person is dropping in the coin each year.

People who profess to know the donor, Gulledge said, claim it's a man.

On the way out of the Kroger, a shopper, Audine Puckett, put money in the kettle.

A gold coin?

No, but every donation – even copper pennies – help make a difference.

"I keep singles and quarters in my purse, just in case," said Puckett, 56, of Howell, Michigan. "It's hard sometimes, because you go by stores and feel like, 'I just gave.' But, I give because I feel blessed. I have so much. I like to give back."

Follow Frank Witsil on Twitter: @fwitsil