A city police spokeswoman said Sunday she could not immediately verify that the trombone had reappeared.
The mom said her son, a fourth-grader at Manoa Elementary School, was "thrilled."
"I'm just overjoyed, I'm delighted," Helen Milligan said. "I don't think anybody ever thought we were going to see that trombone again. So whoever turned that in, I give him a lot of credit."
Aidan put the trombone at the end of his driveway Thursday morning, not wanting to forget it for school. It disappeared while he was in the house preparing for school.
The Philadelphia Orchestra, moved by the story, said Saturday it would donate a replacement horn. It also invited the Milligan family to the season's first family concert, Oct. 27.
Even after the orchestra's offer to replace the trombone, the mother said, she still hoped to see the original turn up.
"I said to them, 'Don't go through with anything, because you never know - someone might turn it in,'" she said. "I thought there was a small chance, but I didn't really think it was going to happen."
She said police told the family that the man who turned it in had no criminal record, and it's unclear whether the trombone was stolen or taken by accident.
"We're not going to ask any questions or anything, we're just glad to have the trombone back," she said. "I just say, 'You know what? I'm just happy he turned it in.'"
Contact Jonathan Lai at 215-854-2771, firstname.lastname@example.org, or on Twitter @elaijuh.