Digging into Mr. Kean’s disclosures from when he was a state senator, a reporter found the address of Mr. Kean’s vacation home in Bay Head, N.J., an exclusive community on the Jersey Shore where Bruce Springsteen also has a home. In May, she paid the house a visit. Advertising fliers were stuck in the front door. There was also a note left in April advising residents to move their cars. It seemed that no one had visited the home for some time.

Mr. Kean also owns a stake in his family’s estate on Fishers Island, N.Y., off the Connecticut coast of Long Island Sound. Like most of the largest homes on the island, the Kean estate sits on a private road behind a staffed guard gate. Pedestrians may walk past the gate, but only residents and their guests are allowed to drive on the eastern part of the island. Reporters arrived by ferry and interviewed people in the island’s public areas, determining that Mr. Kean had not been seen there.

Deep Sourcing

What seemed especially confounding to reporters is the fact that Mr. Kean is so well known in the state; in addition to his own quarter-century tenure in government, his grandfather was a U.S. senator, and his father, Tom Kean, was a popular governor. Reporters sought out former advisers to Mr. Kean’s father, those who ran campaigns against Mr. Kean, Republican Party leaders of counties and municipalities around New Jersey, Democratic Party officials and Mr. Kean’s longtime campaign lawyer. Yet despite dozens of phone calls to people who might have a connection to the congressman, no one had any insight into his condition.

A reporter also called about 20 hospitals in New Jersey, New York, Virginia and the District of Columbia. None had Mr. Kean listed as a patient.

Then, months after his most recent vote in Congress, Mr. Kean gave a phone interview to The New Jersey Globe on May 21. He confirmed that he would run for re-election, but otherwise appeared to reiterate the talking points that his staff had given in his absence. “My doctors are confident that I’m on the road to a full recovery,” he said. “I anticipate that in the next couple of weeks, I’ll return to voting and to the campaign trail.”

On Tuesday, as voters in New Jersey’s Seventh Congressional District cast their ballots, it remains uncertain whether Mr. Kean will emerge in time for the event, and his campaign has no party planned for the evening. With no opposition, his victory is assured. In November, though, he will face well-funded Democratic opposition in what is considered one of the most competitive House races in the country.