Some prescription pill bottles were taken from the apartment but authorities have said little about the cause of the death. There was no sign of foul play or trauma, nor a suicide note, said Ed Winter of the coroner's office.
The young actor appeared in the 1990 film Rocky V as Rocky Balboa Jr, the son of his father's title character. The pair also appeared together in the 1996 movie Daylight.
He is credited with 13 big-screen roles as an actor, while he was a producer on three films, including the 2006 short Vic which he also wrote and directed.
Meanwhile, Sage Stallone's mother has blamed his dental surgery for his death.
Sasha Czack - the first wife of Sylvester Stallone - said her son was taking painkillers after having five teeth removed two weeks before he died.
She told the New York Post newspaper: "I told him not to do that. I've heard about people dying having multiple procedures done to your mouth. Do not have more than one tooth [pulled]."
When asked if he was taking painkillers for it, she answered: "Wouldn't you be?"
Sage, 36, was found dead at his home in Studio City, Los Angeles, last Friday of a suspected drug overdose. The Los Angeles coroner confirmed pill bottles were recovered from his house, but toxicology results could take up to six weeks to be released.
Sylvester, 66, said he was "devastated and grief-stricken" at hearing of his son's death. The pair had become close after filming 1990 movie Rocky V together, in which Sage played his father's on-screen son.
Sage previously said of the filming: "When I was screaming 'You never spent time with me! You never spent time with my mother!' that was true. I was looking into my father's face and really saying that.
"I got a lot of things out. We broke into tears a few times. After the film, everything changed. We'd do anything for each other."
He also said their working together again on 1996's Daylight helped them become closer.
He said at the time: "He'll walk off that set to talk to me now. My father realises that he did okay with me. He did the best he could."
- AFP with Bang! Showbiz