The night before his death, Cory was seen in his room hanging with friends, before hitting the town ... but came back early Saturday morning ... alone.
Cops say "it was not immediately apparent" what caused his death, but no foul play is suspected. An autopsy is scheduled for Monday.
A rep for the actor says they are "in shock and mourning this tragic loss."
Monteith voluntarily checked himself into a treatment center back in April, admitting he had substance abuse issues. It was his second time in treatment.
Prior to that, Cory had been open about his problems with drugs. Back in 2011 he told Parade Magazine, his family and friends were so afraid he "could die" they staged an intervention. He was 19 at the time.
He explained that his drug use consisted of "Anything and everything, as much as possible." Adding, "I had a serious problem."