Los Angeles police officers investigating what they suspected was a "chop shop" -- an illegal operation in which stolen cars are disassembled and their parts sold to unsuspecting buyers -- recently came across car windows bearing a six-digit code.

With one telephone call they found that the windows, along with other parts in the shop, came from a Mercedes-Benz stolen a couple of weeks earlier from a car rental firm.

The police took their evidence to the district attorney's office, where they were told they had an "open-and-shut case" against the chop shop.

What is this six-digit code used to identify the stolen parts? It is the heart of Prevent-a-Theft, a new "passive" anti-auto-theft system.

Although police recognize P-A-T as a "deterrent," D.C. Police Senior Crime Prevention Officer Tony Murray says, "We don't recommend it since there's a charge." He points to the police department's free service in which they engrave the owner's driver's license number on the car, enter the number in their computer and provide a sticker warning thieves of the protective marking.

"The police-provided service can be removed," counters John Forrest, vice president of Chesapeake Cars & Services, Gaithersburg, area distributor of the P-A-T system. P-A-T's six-letter code is sandblasted onto all windows of a car and marked in invisible, permanent ink on other parts of the vehicle.

The code, along with other pertinent information -- color, make, model, vehicle-identification number, owner -- is fed into the company's nationwide computer system, available 24 hours a day to law-enforcement agencies, insurance companies and the National Crime Information Center.

The service, available through new-car dealers, costs $130-$150. Two dozen area dealers currently are offering the service.

"Your car is covered for as long as you own it," says Forrest, "and the service can be transferred -- for a nominal fee -- to the new owner if you sell your car. You also get a kit for putting the same code on your home valuables.

"If your car is stolen and not recovered within 90 days, P-A-T will send you a check for $500, in addition to whatever your insurance company pays you. That $500 payment is guaranteed by Lloyds of London."

What if P-A-T pays the $500 and your car is then recovered and returned to you? "You get to keep the $500."

Some other car-related systems already in existence or being developed by Japanese manufacturers:

* "Rear sonar." Alerts a driver when he has backed his car to an object within 6 1/2 feet of his rear bumper.

* Radar speed-control device. Warns driver of being too close to vehicle ahead.

* Computerized navigation unit. A dashboard device displays a car's position on a map.

And, offered in some 1983 Ford and Chrysler models:

* Audible warning system. An electronically synthesized voice warns of such things as: door left ajar, headlights left on with door open, low fuel, low oil pressure.