"We've long asked people about how they've planned to keep their savings, and for the past few years, we've seen an uptick in people saving cash," said Kimberly Litt, public affairs manager at American Express. This is the first year the company has specifically asked Americans about tucking away cash.

Millennials are even more apt than other generations to go the mattress or freezer route, with 67 percent of those saving cash saying that they hide it outside a bank account.

While banks are still the go-to solution for most consumers, 29 percent say they're keeping at least some savings in cash bills and coins, according to a new survey of 1,820 adults from American Express . Of those holding cash savings, 53 percent are hiding it in a secret location.

The survey also found that about 1 in 4 consumers anticipates a financial emergency this year, and hiding cash at home could be one way people are preparing. "I've also heard of people using it as a budget technique, keeping cash in envelopes set aside," said Litt.

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AmEx didn't ask where, exactly, that cash is stashed, but a 2012 Marist College survey of 1,080 adults found that the most popular place—with 27 percent of the vote—is the freezer. A little less than 20 percent of Americans hide cash in a sock drawer, while 11 percent put it under the mattress and 10 percent secure it in a cookie jar. Another 9 percent keep their cash somewhere else in the house.

"We saw a surge of this back in 2008, when the banking crisis was going on," said security expert Todd Morris, founder and chief executive of BrickHouse Security, a company that sells security, technology and surveillance solutions. The trend has continued in recent years, he said, with more people installing floor and wall safes to store important documents and cash.

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But keeping large amounts of cash in the house is something that makes personal finance experts cringe. "Can't discourage it strongly enough," said Greg McBride, chief financial analyst at Bankrate.com. "That's a recipe for problems down the road."

Primarily, that's because the funds don't have the same loss protections afforded to an FDIC-insured savings or checking account. "Keeping money stashed around the house leaves you at tremendous risk of theft or loss due to fire or some sort of unforeseen disaster," he said.

There are plenty of horror stories, such as the Israeli woman who in 2009 replaced her mother's old mattress, only to learn that it was where she'd hidden her life savings, an estimated $1 million. Or the man in Moline, Illinois, who accidentally donated a suit with $13,000 stashed in a pocket.

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That said, it's not a bad idea to have a little cash at home, for use in an emergency, said McBride. After a major storm, for example, ATM access may be limited or stores' credit card processing systems may not be working. Keep on hand what you'd need to tide you over for a few days, with the exact amount depending on your family's financial situation, he said.

But even then, come up with a decent hiding spot. Thieves know which spots are popular, Morris said. "Burglars definitely know certain places to look, like the bottom of your sock drawers, and underneath your drawers, where it's easy to tape an envelope [of cash]," he said. "If you think about your house and you know where things are, you can think about some places where people wouldn't look." Maybe a tool box in the basement, for example, or a little-looked-at book on your shelf.

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Pick just one place, Morris said. Divvying up cash among multiple spots increases the chance of you forgetting one of the hiding places or otherwise losing the cash (say, by adding the pair of boots where it's stored to the yard sale pile, or throwing out that old hairbrush that's actually a diversion safe). Be sure to tell at least one other person the location of your hiding place, he said. That reduces the chances of an accidental disposal, and also ensures your assets won't be lost if you suddenly pass away.