Of course, Gene said, he understood that people on the block would not want him staying in his Explorer. “I didn’t want to be living there, either,” he said.

He had chosen his car as a refuge, he explained, to stay clear of people who knew him and the hazard of pity. “On some TV series, one of the characters said, ‘The thing that’s toughest about going home is that people want to know: What happened to you?’” Gene said.

He does not have much of an answer for that.

By his account, much of it corroborated in public records, Gene grew up in Boise, Idaho, one of two children. A voracious reader as a child, he discovered chemistry in college, and earned a Ph.D. at the University of Wisconsin-Madison. A grant from the National Institutes of Health paid for his postdoctoral work. At a large pharmaceutical company, he worked with a team on variations of an immunosuppressant, and compounds useful in treating diabetes. Records list him as a co-inventor on at least 44 “composition of matter” patents in the United States and Europe. (The patents are owned by the company.)

Gene said he was married for three years in the late 1990s, and records show that he once owned a house in Princeton Junction, N.J. By 2004, he said, he was unhappy in his job and living on the East Side of Manhattan. When his mother, who was living in Arizona, had a stroke that year, he said, he took a three-month leave of absence, and never went back. Why not? Perhaps, he speculated, the trauma of 9/11 had affected him. And, he said, he had been unable to find a position close to his old rank. “They wanted someone cheap,” he said. “They weren’t going to pay $115,000 for a bench chemist.”

He moved to Brooklyn and took a job with Barnes & Noble in Park Slope that paid about $10 an hour. He also managed to run up $40,000 in credit card debt. How?

“Going out, eating,” he said. “Like I was still making $115,000.”

When he needed a new place to live, a co-worker at the bookstore, Bob Matheson, offered him a ground-floor apartment in the building that he owned with his wife, Diane. Gene moved there in 2008. An $800 rent was discussed but he never ended up paying anything, he said, because of problems in the apartment.

Ms. Matheson led a hermitic existence, and after Mr. Matheson fell ill, Gene said, he recalled accompanying her in 2013 to a medical appointment. She told him she had not been outside “since Bush v. Gore,” Gene said. Mr. Matheson died, and some months later, in fall 2013, Ms. Matheson also died. Under arrangements made by the Mathesons, he was given a year to move out of the house, Gene said, and was offered $10,000 by the estate to leave earlier. Instead, he stayed until he had to go. (Less than a month later, the building was sold for $2 million; renovated, its three apartments are in contract to sell for an aggregate of $4.8 million, according to the website StreetEasy.)