But when bills, bank statements and paycheck stubs also went missing, she began to get suspicious.

"If your mail is missing, it might not just be carelessness of the post office," Barlahan, 29, said. "It might be something worse."

After alerting the post office to her situation, Barlahan learned that she was most likely the victim of mail theft, a problem Sunnyvale has faced for some time. According to Mitch Herzog, a United States postal inspector, there has been an ongoing investigation into Sunnyvale's problem since early April.

"Mail theft is a definite problem," Herzog said. "It ties in with identity theft, which is one of the fastest-growing crimes in the country, and that's why we encourage people to safeguard their personal information."

Barlahan lives in an apartment complex off of Mathilda Avenue, and as is the case with many complexes, mail for all the residents is delivered to one central bank of boxes.

Central boxes like Barlahan's open up in the back of the unit, allowing a postal worker—or thief—access to all the mailboxes at once. Herzog said mail thieves are commonly looking for any letters or mailings with personal information like Social Security and bank-account numbers. Credit-card applications are also a popular target, because a mail thief can apply for a card in someone else's name and run up debts on that credit line.

Also, ATM cards and debit cards—which use Personal Identification Numbers that are mailed to card holders—are easy targets, because all it takes is a typically four-digit PIN number to use the card at any ATM.

Barlahan said she didn't think she lost money as a result of having her mail stolen, but she and her husband cancelled credit cards as a precaution.

Herzog said there are a number of things residents can do to protect their personal information from theft.

The easiest thing is to make sure mail doesn't sit long in mailboxes. People on vacation can have the post office hold all incoming mail, and when they return home, simply show their ID at the post office to retrieve their mail.

If residents expect a bill or important letter but aren't available to pick it up, they can have a trusted friend or family member retrieve it.

Also, by having new checks sent to banks for pickup and applying for direct deposit of paychecks, tax refunds and Social Security checks, residents can make sure their important information never even reaches their mailbox.

Incoming mail is not the only target of mail theft. Discarded credit cards, applications and personal information can be retrieved from recycling bins and trash cans and used by thieves. Herzog suggests people get small personal shredders to destroy sensitive documents before throwing them away.

People can also send bills and completed credit applications from work or deposit them in post office dropboxes rather than mail them from home.

Many personal mailboxes have flags or other indicators to alert people to mail that needs picking up. "Those red flags also let the crooks know that there's outgoing mail in there," Herzog said.

He said no arrests have been made in the latest string of mail thefts in Sunnyvale, but he encourages anyone who suspects their mail has been stolen or sees anything suspicious around mailboxes to contact local post offices or public safety officers immediately.

The postal inspection office in San Francisco has the only 24-hour hotline for residents to call. That number is 415.778.5900.