What do you do with your boarding pass after you settle into your airplane seat? If you've followed the advice of our Savvy Traveler, Rudy Maxa, you keep it to make sure you get credit for your miles. But many of us leave it on our seat, tuck it in a seatback pocket, or toss it in a nearby wastepaper basket when we deplane. VERY bad idea, says Rudy.

There's information on that little piece of paper, and these days, information such as your name and frequent flyer number aren't what you want strangers to have. Recently, a resourceful reporter from the Guardian newspaper in London took a boarding pass he found on a British Airways flight to a computer security expert and asked him what he could find out about a perfect stranger who'd discarded it.

Using the passenger's name, the security expert entered British Airways' web site, typed in the frequent flyer number and, in a matter of moments, knew the passenger's passport number, nationality, and date of birth. By the way, the trespasser could have changed all those details if he'd care to in that customer profile.

Then, using that information and public data bases, the expert and the reporter quickly learned the passenger's home address, who lived with him, where he worked, his educational background, and how much he'd paid for his home. All great building blocks for a clever identity thief.

For years now, Washington has been trying to figure out how to gather and store sensitive personal information in order to pre-screen airline passengers--the better to weed out terrorists. The Transportation Security Administration now also wants to know where passengers are coming from and going to as avian flu threatens us. And it wants to be able to share--and trade--personal information about with airlines as well as other government agencies.

Knowing details such as our credit records and work history, some argue, will allow authorities to separate the bad guys from the good guys. But it also means more computers will be filled with intimate details of our lives. That makes protecting your identity even more important.

It's not just airline boarding passes that can be the key to a treasure chest of information about you and your life. How many times have you, Mr. or Ms. Business Traveler, left your rental car contract in the glove compartment of a car as you've dashed off to catch a flight? I looked at mine the other day, and it contained my full name, home address, half my American Express card number and an airline frequent flyer number. And when I filled up my rental car with gas, I looked closely at the receipt and saw my name and half my credit card number on it.

Here's another Achilles' heel: Ever left a magazine--one with your name and home address on a subscriber's label--on your airline seat as you deplaned? It's an open invitation for someone with larceny in their heart to begin to steal your identity. And luggage tags are another source of information. Either cover your tag or just put your first initial, last name and phone contact on it.

We tend to think we're anonymous when we travel. But just because no one recognizes you on the street doesn't mean you aren't leaving big, fat clues about who you are, where you live . . . and how you can be ripped off.

From Saint Paul, I'm Rudy Maxa for Marketplace.

May 17, 2006