The damage created by extramarital affairs is well known, but financial infidelity can wreak just as much, if not more, havoc.

Financial infidelity consists of one partner in a relationship concealing assets or debts or taking some other secretive action that hurts the couple's finances, Lili Vasileff, founder of Divorce & Money Matters, a divorce financial planning firm, tells Bankrate.com.

The problem apparently is widespread. One-third of people who have joined finances with their partner, say they have either committed or endured financial infidelity, according to a January survey by the National Endowment for Financial Education.

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Bankrate identifies several signs of possible financial infidelity.

• Missing or misdirected documents,
• Getting cut off from a joint credit card,
• Seeing no activity by your partner on a card you both usually use,
• Receiving statements from a financial concern you don't recognize,
• Interception of bills and statements by your partner,
• Missing cash,
• Tales of financial woe from your partner,
• Unusual generosity from your partner.

As for extramarital affairs, a recent survey conducted by a U.K. retail company calculated that the average affair lasts six months and costs $444 a month, or $2,664 in total, CNBC.com reports.

That $444 consists of $123 for hotel rooms, $162 for food and beverages, $54 for gifts, $69 for movie tickets and $36 for "other" date activities.

But experts say the costs can be much higher. "Secret cell phones, airline tickets, secret credit cards, hotel rooms, the costs can be astronomical," Ramani Durvasula, a licensed clinical psychologist, told CNBC.com.

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Related Stories:

• This No. 1 Marriage Killer Lurks in 100% of Relationships
• How Much Does That Extramarital Affair Cost? At Least $2,664
• Financial Infidelity Can Ruin Your Relationship