MORRISTOWN, N.J. -- Alarms reverberated in many corporate boardrooms when Sidney J. Reso, president of Exxon Co. International, vanished April 29, his car left idling in his secluded driveway 250 feet from his house.

The mystery has companies taking extra care to protect their top people.

"We're concerned about copycat cases. We're advising executives to be a little more careful than normal," said E.C. "Mike" Ackerman, a Miami-based former CIA agent who consults for top multinational corporations. "This is a very puzzling case."

No one can say for sure what happened to Reso, an Exxon employee for 35 years and, for the past four years, president of the subsidiary that oversees Exxon's explorations and production outside North America.

A group called the Rainbow Warriors contacted the company and told it to "have a lot of money ready," but it offered no proof it had Reso. No one had ever heard of the group before.

Foul play is more likely in such places as Colombia, Peru, Brazil, Pakistan, India or the Philippines -- not the exclusive, manicured suburbs of northern New Jersey.

A 10-year-old study done for the Conference Board, a New York-based research group, noted that 55 percent of the 567 Americans kidnapped overseas from 1970 to 1978 were business executives.

But in this country, Ackerman said domestic nabbings are "almost negligible." Patrick McGuire, who did the study for the Conference Board, estimates there are fewer than a dozen each year.

"The number of times a high-profile executive is actually kidnapped for ransom in the United States is extremely low. Kidnapping is one of least successful crimes you can commit," McGuire said.

"Executives have more to fear from dissident stockholders."

Alan Bell, a former commando in the British Royal Marines and manager of Intercon Security Ltd. in Toronto, warns executives to take extra care driving to work, parking their cars and booking airline tickets and hotels.

Even if the incidence is low, corporate bigwigs remain targets for crusaders, criminals or crazies looking to make a fast buck or broadcast a statement.

"It's a subject no one wants to talk about," Bell said. "You won't find any facts or figures. No one will produce statistics. No one is going to admit if it happens because they don't want bad publicity and they don't want to give people ideas on how to make big money very quickly.

"It happens more frequently than it surfaces in the media, though. A lot of this work is done behind the scenes," Bell added.

Most corporations have insurance policies that pay ransom if an executive is kidnapped, or security policies to prevent kidnapping, said Terry Van Gilder, executive vice president of the insurance provider Chubb & Son Inc.

"It is certainly safe to say those corporations have security directors who pay a lot of attention to incidents like" Reso's, Van Gilder said.

Patricia Reso told the Star-Ledger of Newark that she found the car with the engine running, the doors closed, and her husband's overcoat and briefcase inside. There was no sign of a struggle. Police in helicopters, on horseback and on foot searched the area, aided by a police dog.

"Nobody's amply demonstrated they have him in captivity -- no picture, no tape, no proof," said W. Michael Murphy Jr., the Morris County prosecutor who is leading a 50-member task force augmented by the FBI.

"It's important for us to know he's in someone's hands and he's well," Murphy said. "That's if he indeed was abducted. Voluntarily absenting oneself from home, family and job is something we have to consider."

In the meantime, the case revolves around speculation and unanswered questions.

"At this point, it's almost as if it's suspended in mid-air," said Robert Kobetz, a crisis management consultant in Berryville, Va.

Some executives who have been abducted in recent years:

Victor Samuelson, an executive with the Exxon subsidiary Esso-Argentina, abducted by Marxist guerrillas in Argentina in December 1973. He was drugged and held for 144 days before a record ransom of $14.2 million was paid and he was released.

Kenneth Bishop, an executive with the Texaco subsidiary Texas Petroleum, abducted March 1983, in Bogota, Columbia, by gunmen who identified themselves as the People's Revolutionary Organization. His two bodyguards were killed. Bishop was freed after his family reportedly paid a ransom of several hundred thousand dollars.

Michael Stewart, an executive with the Colombian subsidiary of Houston-based Tenneco, kidnapped August 1985 by six leftist guerrillas in Bogota. Stewart was held for four months; Tenneco officials declined to give any details about his release or ransom.

Stephen B. Small, a media heir and former executive in his family's broadcasting company, Mid-America Media Group, kidnapped September 1987 in Kankakee, Ill., and held for $1 million ransom. He was buried alive in a homemade plywood box equipped with candy bars, water and a breathing tube. But something went awry and he suffocated. In 1988, two people were convicted of murder and kidnapping; one was sentenced to die, the other to life in prison.

Jake Gambini, chief of operations for General Pipe Services of Bogota, abducted June 1988 at gunpoint by five members of a left-wing guerrilla group demanding a $2 million ransom. Gambini was released after 138 days in captivity. His family paid an undisclosed ransom.

John Grundhofer, chairman of First Bank System, abducted at gunpoint November 1990 in Minneapolis and forced to make a ransom demand from his car phone. But the ransom wasn't paid and Grundhofer escaped unharmed after he was bound, placed in a sleeping bag and left at the bottom of an embankment.

Michael Barnes, vice president of the Unocal Corp. subsidiary Philippine Geothermal, abducted at gunpoint January 1992 by communist terrorists who demanded $20 million in ransom. No money was paid, and Barnes was rescued 61 days later by Philippine police commandos after six raids that left 12 suspected kidnappers dead.