Little more than a novelty a decade ago, anti-theft auto security systems are mushrooming into sales of more than $500 million each year, reports an automotive industry journal.

And with thefts of and from autos increasing every year, sales of these security devices are currently growing at a rate of 25 percent each year, notes Automotive Electronics News, a newspaper that tracks the use of electronics systems in the automotive industry.One of the more exotic anti-theft systems available operates on radar. It sounds a warning if the airspace of the passenger compartment is invaded, unless the owner deactivates a hidden switch.

Another is a system that "sounds off' and denies any entry if it detects "scanning" - a burglar's attempt to send a series of radio signals in an effort to find the correct code to open the door.

Reflecting the advances made by security systems, seven states - New York, Massachusetts, Michigan, Kentucky, Illinois, Rhode Island and Texas - mandate discounts on insurance to drivers whose vehicles carry anti-theft systems.

The rebates range from 5 to 35 percent, depending on whether the alarm is passive, or active - the kind that can be armed by the driver.