DETROIT (AP) _ A Florida motorist concerned about thieves used to keep his sports car chained to two palm trees in his yard each night.

One moning he noticed the rear bumper was chained where the front bumper had been. And there was a note on the windshield: ''When we want it, we'll come back and get it.''

A true story, swears David Manly, a vice president at LoJack Corp. in Needham, Mass.

Manly maintains that devices like ear-piercing alarms and metal shanks which fit across steering wheels are a mere annoyance for today's sophisticated thieves. LoJack and a growing number of other companies concentrate on systems that get stolen cars back - and intact.

Sales of auto-theft retrieval devices accounted for about $5 million of the $365 million spent on auto security systems in 1989, said Ed Hester, vice president of the durable goods division of the Freedonia Group, a Cleveland- based producer of industry research reports.

But Hester predicts the overall auto security market will approach $800 million by 1994, with 17 percent annual growth. He said sales of retrieval systems should grow at a 38 percent annual rate.

Five-year-old LoJack - its name is a takeoff on hijack - so far has sold more than 100,000 of its $595 homing-device units in the six states where it has agreements from police agencies to go after stolen cars. Those states are: Massachusetts, California, New Jersey, Michigan, Illinois and Florida.

LoJack gives police agencies on-board equipment to pick up the signal from a LoJack transmitter hidden in cars before they were stolen. About one in four car thieves has been caught with a hot car because they didn't know about the trasmitter, Manly said.

''We've had a big success rate,'' said Bob Weisz, a Los Angeles Police Department officer. ''We've discovered in the last year that we don't have enough of the tracking units because of the cars' downtime.''

Forty-seven of 48 police departments in Los Angeles County as well as the sheriff's department and the California Highway Patrol make up a network of LoJack users that has recovered 75 of 85 LoJack-equipped stolen cars. In all but one case, Weisz said, the cars had only minor damage.

''The county loses about 129,000 cars and recovers 114,000 a year,'' Weisz said, although a significant number is just the remains of a vehicle.

Police agencies require that a vehicle be reported stolen before they activate their tracking equipment. Some LoJack competitors operate by continuously tracking movement of the car. Any unauthorized movement prompts a call to police from a central command.

Code-Alarm Inc., of Madison Heights, Mich., began selling its Intercept system last year. It includes an alarm and a cellular phone, which automatically dials the Intercept control center if the car is moved without its key or if its alarm is bypassed. With the phone, the system costs about $1,400 plus installation and a $15 monthly service fee.

International Teletrac Systems Inc., of Inglewood, Calif., sells a system that combines the homing device with the centralized tracking system using its own frequency and towers it has constructed. It sells for $899 with a monthly $15 fee.

The retrieval systems have substantial up-front costs, but many insurance companies will provide discounts on premiums ranging from 5 percent to up to 35 percent, depending on the state and the mix of anti-theft equipment installed.

LoJack, Code-Alarm and Teletrac all guarantee at least a partial refund of the cost of their tracking devices if the car is stolen and not recovered in a certain period of time.

In advertising, LoJack uses the testimonials of customers whose cars were stolen and recovered.

One of them, Peter Bernson, who owns two video rental stores in the Boston area, had his Camaro convertible stolen and recovered in January 1990.

''It was in the hands of three people who were about to make it into about 18,000 pieces,'' said the 42-year-old Bernson.

The car was recovered with only minor damage and police made two arrests.

Auto theft, chop shops and resale of scavenged parts are an $8 billion annual business, according to the National Automobile Theft Bureau in Palos Hills, Ill.

In the Detroit area, one of the nation's hotbeds for auto theft, retrieval devices are making a difference, Michigan State Police Lt. Sandra Miller said.

Miller is part of a consortium of law enforcement agencies that targets commercial auto theft. The group has one LoJack tracking-equipped car, an undercover vehicle used in surveillance of chop shops.

Miller says she's especially fond of the Teletrac system, which works regardless of whether the car owner is aware of a theft. Because most cars stolen in the Detroit suburbs end almost immediately at chop shops in the city, the head start is critical, she said.

While recovery systems have won praise among law enforcement officials, the National Automobile Theft Bureau, which was founded by the insurance industry 79 years ago, maintains most motorists need not go to the expense of investing in them.

''Lock the vehicle and take the keys,'' said Paul Gilliland, the group's education and training coordinator. ''It's surprising how many vehicle thefts occur today that the keys are found or left in the ignition.''

Gilliland said automakers also are doing their part in preventing theft.

General Motors Corp., for instance, has a Pass-Key system requiring a matching of miniature transistors hidden in the key and the car body as well as the key to the lock. The system is standard on Chevrolet Corvettes, Camaros and Pontiac Firebirds, all frequent targets of car thieves.