Hunt Continues for Vanished Oil Executive : Mystery: His car was found with its motor running. Authorities are unsure if ransom note is genuine.

It’s been 16 days since Sidney J. Reso, the president of Exxon Corp.’s International Division, bade goodby to his wife, left his expensive home in a New Jersey suburb for a routine 10-minute trip to his office--and vanished.

So far, an intensive manhunt by police and the FBI has turned up few clues, and there are fears he may have been kidnaped, perhaps by an environmental group.

After he failed to arrive at Exxon Co. International in nearby Florham Park, N.J., on the morning of April 29, Reso’s car was found with its motor running at the end of his 250-foot-long driveway in suburban Morris Township, N.J. The door was open on the driver’s side, and his overcoat was still inside.

Exxon has received a ransom note from a group calling itself “Rainbow Warriors,” but investigators have not been able to determine whether the note is a hoax.

The company has not received any pictures or videotapes indicating whether Reso, a 57-year-old Exxon corporate veteran, is being held captive.

“It remains a mystery,” said an official of the Morris County prosecutor’s office, which is deeply involved in the case.

Rainbow Warrior is the name of an environmental protest ship that French security agents sank in New Zealand in 1985. Greenpeace, the international environmental group which owned the vessel, has denied any knowledge about Reso’s disappearance.

“If indeed there is an environmental group, they should know his feelings and concerns about the environment,” Reso’s wife, Patricia, said in a televised public plea recently. “If they did, they would not have chosen him.”

In a comment to the Newark Star-Ledger, Patricia Reso also said she was concerned that her husband did not have his medication with him. She said Reso had suffered a heart attack several years ago.

Exxon has announced it will pay a “substantial reward” for Reso’s safe return. The company did not specify the amount.

Exxon said its decision to offer the reward was made to solicit information and said that special telephone numbers had been set up to receive confidential calls. An Exxon spokesman Thursday declined to comment on whether any significant information had been received.

In her latest television plea Wednesday night, Patricia Reso described her husband as a courageous man. “He’s got a strong faith. . . . He’s honest and he’s good,” she said.

Patricia Reso explained that she discovered the car in the driveway after her husband’s secretary telephoned to say that a neighbor had seen the auto.

Immediately after Reso’s disappearance, police, using dogs and helicopters, scoured the area around the executive’s home. When that proved fruitless, investigators began the tedious process of scrutinizing all aspects of his business and personal life, including his appointments and his travels on behalf of Exxon.

Investigators stress they have no concrete proof the top Exxon executive was kidnaped. At a news conference last week at which the existence of the ransom note was revealed, Morris County Prosecutor W. Michael Murphy Jr. stressed “it is the desire of all parties that Mr. Reso be returned unharmed in an expeditious fashion.”

Murphy said the disappearance was still being classified as a missing person case and declined to reveal what progress had been made.

Reso, an engineering graduate of Louisiana State University, was named president of Exxon’s international operations in 1987 after serving in a number of overseas assignments with the company.