Law enforcement tells TMZ ... officers were called out around 11:30 AM Easter Sunday to a motel in Orange County, arriving on the scene after staff told them they stopped by Josh's room after his checkout time -- as we're told he'd failed to let anyone know he'd left.

We're told Josh's body was found in his room -- with drug paraphernalia and residue also discovered ... which is why police are treating his death as a possible OD.

Cops say foul play is not suspected at this time. However, our sources say nothing can be confirmed until the results from the toxicology report come in.

Lauri announced her son's death on Saturday, touching on his substance abuse struggles in her tribute, but highlighting his positive attributes. She thanked those who'd tried to help Josh ... noting she'd heard moving stories from parents who were similarly situated.

Josh's struggles were well-documented on 'RHOC' and beyond -- which included several run-ins with the law. Josh is survived by his daughter Kennedy, who he shared with his ex-wife Hannah. He was only 35.