A halt in direct deposits, for example, might indicate a customer has lost his job. If the customer has a high behavior score and a high FICO score -- which are good indicators that he or she will find another job soon, Gordon said -- the customer might be offered a payment holiday, with the missed payments being added to the end of a loan. If his scores aren't so high, suggesting a protracted job hunt, the bank could suggest a workout plan or take other measures to reduce its risk.

Banks also may use depositor behavior scores to extend credit to people who may not have a credit history, such as young people or immigrants, Gordon said.

"The score was developed to help the underserved or inconsistently served," Gordon said.

3. Income estimators

The Credit CARD Act of 2009 requires bankers to assess applicants' ability to pay a credit card account, and they're allowed to use analytical tools that guess at potential customers' income.

And "guess" is the operative word, because income-estimation tools are used to extrapolate your pay from factors such as the size of your mortgage and your available credit lines. The estimations aren't precise: Experian said last year that about 85% of the incomes it estimates around $35,000 are in reality only somewhere under $50,000 and that 15% are actually above $50,000.

It's not that your income is necessarily all that secret. Huge employment databases exist that list people's names, employers and salaries. But not everybody is in these databases, the information can be outdated, and using an income estimator is typically cheaper, so bankers increasingly use this tool, along with others, to evaluate you.

4. Wealth estimators

Another factor that determines your ability to pay is your overall wealth -- your home equity, savings and other assets. But bankers are also interested in ferreting out this information for marketing purposes, so they can reach out to potential customers and pitch more-appropriate products to the customers they have.

Credit bureau Equifax made a big move in this arena when it bought IXI in 2009. About 100 banks and other financial institutions submit information about individuals' investments and assets to IXI's database, which represents more than 40% of individual invested assets in the U.S. The data are anonymous, so the individuals involved aren't personally identified, but it's aggregated by ZIP code to give banks an idea of average wealth levels. The database is also used to construct models to estimate individual wealth.

5. Collection services

As you've learned, bankers have software that can help them assess whether a missed payment was likely an oversight or if you're really in over your head. Once it's clear you've stopped paying, bankers can use other tools, including software that predicts how likely they are to be able to collect from you and programs that monitor your financial situation, looking for signs of improvement -- such as a new employer or debt levels that start to decline.

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Credit bureau Experian, for example, touts its Collection Triggers program with the tag line "When nonpaying customers resurface, be the first to know." The program offers daily monitoring of credit reports for a customizable set of triggers including new contact information or indications of increased ability to pay debts, which can help subscribing lenders "be the first to the (debtors') door for wallet share" -- in other words, the first to ask for repayment.

Now, the use of this and other monitoring tools isn't universal, and banks (and credit unions) often use the same tools in different ways. But it's a safe bet that your bank is using several tools to keep tabs on you and to extrapolate the various risks and opportunities presented by your behavior. That may be good news if you carefully manage your accounts, but woe betide you if you mess up. Remember, Big Banker is watching.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.