Rob & Michele Reiner's Son Nick Was Missing for Hours Following Their Murders

Nick Reiner -- the son of Rob and Michele Reiner, who's been arrested on suspicion of murdering them -- was missing for hours after his parents were found with their throats slit inside their Brentwood home ... TMZ has learned.

Law enforcement sources say ... after Rob and Michele's daughter Romy found their bodies, she noted to police Nick had been living there and was possibly behind the murders.

When police responded to a call for a death investigation around 3:40 PM Sunday, Nick was nowhere to be found. Romy was the one who called 911 and told police her brother was "dangerous" and should be considered a suspect.

Romy told police Nick was not there when she arrived, insinuating he fled the scene. Police finally located and arrested Nick at approximately 9:15 PM Sunday.

Nick is currently in police custody, held without bail.