If you're being hounded by a debt collector or repo man, don't be surprised if they seem to know an awful lot about you.

That's because companies in the business of tracking people down often buy a variety of information to aid in their efforts.

TransUnion's subsidiary, TLO, for example, says on its website that it will sell reports to debt collectors that include your Social Security number, cell phone number, details of the property you own, even the names of the friends and family you hang out with.

TransUnion said that all of its customers go through a rigorous credentialing process before getting access to any data.

Under federal law, your Social Security number is fair game to be sold to debt collectors since it's being used for credit-related purposes. Although, in many cases, debt collectors already have your Social Security number.

And that's just the beginning. Some data brokers collect information gathered by license plate scanners to track "vehicle sightings," which provide the physical location of cars. They then sell that information to repossession companies and other firms.

In a report last year, the American Civil Liberties Union said these private databases raise "serious privacy concerns," in part because they aren't subject to any data security or privacy regulations.

Related: Big Data knows you're broke