Investigators were wearily studying their umpteenth shipping manifest when - bingo! - there it was.

A Mercedes-Benz, loaded on a ship steaming out of the nation's busiest port and headed for Hong Kong, matched the identification number of a sedan reported stolen from a northern California shopping mall.The discovery launched an international investigation that has netted 72 suspects and recovered millions of dollars worth of Lexus, BMW and Mer-ce-des-Benz cars destined for Asia. Millions more in Landcruisers, Broncos and Su-bur-bans headed for Latin America have also been traced.

The Associated Press first reported in 1992 that the vast crush of legitimate traffic at more than 300 American ports was working to smugglers' advantage because inspectors can search only a minuscule portion - perhaps 1 percent - of vessels leaving American waters.

Cars, motorcycles, computers, hi-fi gear, software, guns, appliances - all are stolen here and smuggled to eager buyers around the world.

But investigators have focused only relatively recently on the steady, lucrative traffic that dwarfs all the rest - stolen high-end, luxury automobiles, a commodity estimated by insurers to be worth $1 billion or more a year.

Government and insurance industry investigators say more than 200,000 "stolens" slip out of the United States annually - perhaps 550 vehicles a day. Most believe that figure is low.

"It's probably higher, and the truth of the matter is nobody knows how many cars are going out," said Shan Haider, claims investigation manager for Farmers Insurance Co.

"It's like looking at the tip of an iceberg - you wonder how much more is underneath the water," said Mark Stowell, a National Insurance Crime Bureau investigator assigned to the Long Beach-Los Angeles port.

About 160,000 vessels enter or leave West Coast ports annually, most of them freighters. Aboard are millions of boxy, sealed cargo containers, "intermodal" units that can be carried on a ship, loaded on a railroad flatcar or plopped onto a tractor trailer.

Most containers leave the country untouched, their contents verified by voluminous paperwork but not by the per-sonal scrutiny of a federal agent.

"It is just physically impossible to search every one of those ships," said Jeff Benzing, a spokesman for NICB. "You'd probably have to have a million customs agents just to do it."

Most stolen cars leave the West Coast for Asia and Latin America, where they fetch the highest prices. Some go through Miami, a popular export point for the Caribbean.

Foreign criminals hire gangs to steal specific cars. In one case, U.S. investigators traced 14 stolen luxury cars to a Thai who had put in an `order' with an Orange County Asian gang.

The scam is simple. "Straw buyers" purchase luxury cars in the United States, drive them for a couple of weeks, then place them in sealed shipping containers. They're loaded on a ship, then unloaded in Asia some 10 weeks later.

Sometimes paperwork correctly identifies the container's contents as a car; other times the paperwork is bogus or it describes the contents as appliances, textiles or motor oil.

If the container is opened, it may appear to be filled with legal products. The car is behind a false wall.

The buyer reports the car stolen only after it leaves the country. He collects the insurance payment, which goes back to the ring that supplied the cash for the original purchase.

The scheme's success depends on two things - the rarity of a physical search and the delay in reporting the "theft" until the vehicle has been shipped.