This was sent to me by a retired LEO from Scott County who has verified it with some LEO's who are still on the force. They have heard of accounts where this has been used.

How to Lock Your Car and Why
I locked my car --- as I walked away I heard my car door unlock I went back and locked my car again three times. I looked around and there were two guys sitting in a car in the fire lane next to the store. When I looked straight at them they did not unlock my car again.
How to lock your car safely -
While traveling, my son stopped at a roadside park. He came out to his car less than 4-5 minutes later and found someone had gotten into his car, and stolen his cell phone, laptop computer, GPS navigator briefcase.....you name it....
He called the police and since there were no signs of his car being broken into - the police told him that there is a device that robbers are using now to clone your security code when you lock your doors on your car using your key-chain locking device..

They sit a distance away and watch for their next victim. They know you are going inside of the store, restaurant, or bathroom and have a few minutes to steal and run. The police officer said to manually lock your car door-by hitting the l ock button inside the car, that way if there is someone siting in a parking lot watching for their next victim it will not be you.
When you hit the lock button on your car upon exiting...it does not send the security code, but if you walk away and use the door lock on your key chain - it sends the code through the airwaves where it can be stolen. Something totally new to us...and real.
Be aware of this and please pass this note on...look how many times we all lock our doors with our remote...just to be sure we remembered to lock them....and bingo someone has our code...and whatever was in the car...can be stolen.
Snopes Approved.Please share with everyone you know... Good information!!!

This practice is easily ten years old.

This is also true for older garage door openers. This practice led to widespread use of code hopping in remote-controlled security systems. Like Macphisto said, this is not a new thing, and I am not sure if there are any current model alarm systems on the market susceptible to it. If you are worried about it check the documentation on your system an see if it has code hopping.

Yep. And key fobs these days use rotating codes. Every time you press the button, it advances to the next code. The car will accept any of the next 255 codes, (in case you push the button a couple of times while out of range), but it won't accept already-used codes.

So this shouldn't work on new key fobs.

Of course, you can lock people out, if you press their key fobs 256 time. If you do so, you might force someone to (oh, horrors!) actually use their key.

http://auto.howstuffworks.com/remote-entry2.htm

Uh, don't see it on Snopes...

You people must be very wealthy. My 93 Civic does not even have power door locks…..

I am not sure of the age of this story, but it was confirmed by a friend of my parents in law, the retired LEO from Scott County. As for the snopes approved part, I took the LEO's word more so than the snopes approved portion of the letter. I apologize if this is old news. My 03 Civic has power locks, but no key fob so I actully have to use my key every time to unlock my car.

I doubt very much that the car manufacturers added the complexity of rotating codes until after there'd been publicity about the vulnerability that using a single codes created.

In other words, it was likely it was because of stories like yours that the problem was fixed in later-model cars.

The only way I can arm my car alarm is from my key fob.

My alarm on the car works only off the key so no problem with my cars.

Layers of security, folks. Lots of ways to break into cars with broken glass being the fastest and easiest. There are lock guns. There are tryout keys. There are picks. There are mechanical unlocking devices that are variations on the theme of a coathanger. There are electrical and radio attacks like the one described.

Leave nothing visible that is of value to a thief or that telegraphs the possible presence of hidden valuables, park in a secure location to the extent feasible, etc.