Woman reviewing credit report © Jose Luis Pelaez, Blend Images, Getty Images

Mari Frank couldn't figure out how an identity thief got her Social Security number and birthdate -- until she noticed that her credit report had been viewed by a company she never heard of.

"I had to go through my credit report with a fine-tooth comb," said Frank, a California attorney who has become an expert on identity theft since her run-in with it in 1996. "There was an inquiry there from a mortgage company -- I wasn't buying a house, I hadn't refinanced."

Though most credit report inquiries are legitimate, there are several scenarios that can easily lead to it being read by someone who shouldn't. The information contained there can be used to steal your identity, damage your credit or, at the very least, invade your privacy.

A limited number of entities can legally view your report, under provisions of the Fair Credit Reporting Act. Those include a company planning to offer you insurance or credit (this includes landlords and utilities), a company with which you have an existing debt, an employer or prospective employer, or a court, state or law enforcement agency. You do have the option to voluntarily give permission to anyone you choose to see your report and, of course, you can see it yourself. That's it.

But there are a surprising number of ways your credit report could wind up in the wrong hands.

In Frank's case, it turned out to be a clerk at a law firm that was representing a mortgage company who misused her credit report. The firm had legitimate access to the credit reporting system, but the rogue employee used it to set up credit cards and get loans in Frank's name. It took 11 months to clear up more than $50,000 in fraudulent debts that were attributed to her, Frank said.

Illegal viewing of your credit report should be taken seriously, says Cary Flitter, partner at consumer law firm Flitter Lorenz in Philadelphia. "When people get your credit report illegally, it's as if they had broken into your house, jimmied open your desk and looked through all your manila folders. Here's your mortgage status, here's a collection letter you received. It's all in one place and organized."

The Federal Trade Commission received more than 279,000 complaints about identity theft in 2011, which topped the complaint list for the twelfth year running. Some portion of those are the result of credit report abuse, but the scale of the problem is difficult to determine, an agency spokesman said. It is rare that a victim can tell -- as Frank did through painstaking research -- that their credit report was misused. "People wouldn't know if someone is accessing their credit report who shouldn't access it," said Mitchell Katz, senior public affairs officer at the FTC.

Here are the most common scenarios in which your credit report might be viewed illegally:

Love gone wrong

If you're in the midst of a divorce or a child custody battle, your ex, his or her lawyer, or a private investigator working for that lawyer might pull your credit report to check on your activities and look for accounts or assets you may be keeping secret. Legally, they have no right to do this unless they've obtained a court order. If they haven't, "you absolutely have the right to sue them as this violates the Fair Credit Reporting Act," says personal finance author David Bakke.

There's another possible scenario even before lawyers and private investigators get involved: Your spouse or partner could decide to take a look at your credit report in search of secret accounts that may indicate infidelity (or at least financial infidelity). If he or she has access to your Social Security number, bank or credit card account numbers and is likely to know the answers to your security questions, it may be easy enough to request a credit report or view one online while pretending to be you.

What to do about it: Strictly speaking, a spouse or partner who does this has committed identity theft, says Rod Griffin, director of public education at credit bureau Experian. You'd be well within your rights to file a police report, he says, but depending on the situation, you may not want to go that far. "It's a difficult challenge," he says. "You'll have to have some very difficult discussions with that person."

A better option might be to freeze your credit report with each of the three bureaus, particularly if you know that separation or divorce is on the horizon. Freezing your report renders it inaccessible for most uses, although a company with which you already have debt will still have access, as will courts and governments and private investigators -- but only when performing government-sanctioned background checks. The cost to freeze a report is set by state law and is usually quite low.

In most cases, you can "thaw" a report temporarily, for instance, while shopping for a car loan, and then refreeze it when you're done. Freezing and thawing can be done online, by password.

In a divorce or child custody situation, another possible scenario is that your spouse or spouse's attorney will actually request that you obtain your own credit report and provide it voluntarily. In this situation, your smartest move may be to comply, assuming you have nothing to hide. The other side may well be able to get a court order demanding it anyway, and your refusal will likely arouse suspicion. "If there's hesitation in providing a credit report, that's smoke," says Lili A. Vasileff, a certified financial planner who specializes in divorce. "It means there may be something in there that's unknown to your spouse."

More from CreditCards.com: