"By 1979, he was managing the lots, a job that came with the keys to an inconspicuous entry and an empty concession stand in left field..."

"Mr. Garvey estimated that the space, whose ceiling sloped down with the 300-level seats above it, was about 60 feet long and 30 feet wide. He created a hallway of cardboard boxes to disguise the apartment from the door. 'I open the door and it looks like a storeroom,' said Mr. Bradley, the former Eagle. 'But if you walk down between the boxes, it opened up into one of the neatest apartments I think I’d ever seen.' There was AstroTurf carpet, a bed, some seating, a coffee table and lamps. Devices included a toaster oven, coffee maker, space heaters and a stereo.... Mr. Garvey called it 'cozy,' with 'everything a guy would want.' Bathrooms were across the hall, employee showers downstairs.... In his book, Mr. Garvey describes 'an off-the-wall South Philly version of "The Phantom of the Opera,"' including encounters with the Eagles coach Dick Vermeil, the Sixers legend Julius Erving and the Phillies pitcher Tug McGraw.... 'It was euphoric.... It was like a form of meditation for me. It just — it helped me a lot.' He hid in plain sight: Everyone knew him, he said, and his job gave him a reason to be around at any hour, every day of the week. 'It was right in front of their eyes, they just couldn’t believe it... I wouldn’t believe it myself. The disbelief is the key to how I got away with it.'"

From "Man Says He Lived in Philadelphia’s Veterans Stadium for Years/Several people corroborated parts of the account of Tom Garvey, a Vietnam veteran and former stadium employee who described his 'secret apartment' in a recent book" (NYT).

Here's Garvey's memoir, "The Secret Apartment: Vet Stadium, a surreal memoir."

ADDED: Here's The Philadelphia Inquirer article on the subject. It has some additional details:

At night when he was by himself, Garvey would sometimes roller skate around the concourse. “To roller skate around what would be the equivalent of a 10-story building and to look out and see the city was like meditation after a while,” he said.

Once, Garvey went to sleep during a Phillies doubleheader in 1980. A rain delay caused the last game to stretch well into the early-morning hours. When Garvey awoke in the middle of the night, he went out to watch it in flip flops and a bathrobe with a warm cup of coffee.

“There were less than 200 people scattered around,” he said. “They didn’t want to know why I was there in a bathrobe and flip flops, they just wanted to know where I got a hot cup of coffee because the concession stands closed hours ago.”