Nick Reiner — the son of Rob and Michele Reiner, who’s been arrested on suspicion of murdering his parents — stuck out like a sore thumb at Conan O’Brien‘s Christmas party … and he was acting out of place.

Sources at the party, which went down Saturday night at Conan’s, tell TMZ … Nick seemed out of place based on how he was dressed and how he was carrying himself.

We’re told Nick showed up to the very formal party in a hoodie, while everyone else was wearing nice suits and dresses.

Our sources say Nick roamed around in his sweatshirt, didn’t interact with people, kept to himself, and was very reserved … all of which made some folks feel his behavior was odd.

Nick did talk to one person, though … as we first reported, Nick and Rob got into a “very loud argument” at the party, loud enough for many others to overhear. Rob and Michele left the party after the argument, but it’s not clear when Nick left.

The day after the party, Rob and Michele were found in their Brentwood home with their throats slit.

Nick was arrested and booked for murder … he’s in police custody, being held without bail.

Nick’s sister, Romy, found their parents’ bodies and told police Nick had been living with his parents, was “dangerous,” and should be considered a suspect.

Nick Reiner is on suicide watch after being arrested on suspicion of murdering his parents, Rob and Michele Reiner … TMZ has learned.

Law enforcement sources tell TMZ … Nick is on suicide watch and in administrative segregation at Twin Towers Correctional Facility in L.A. … while he’s being held without bail.

As we reported … prior to Rob and Michele’s stabbing, Nick got into a “very loud argument” with his father at Conan O’Brien‘s Christmas party Saturday night. We’re told the shouting match was loud enough for many others to hear, and that Rob and Michele then left the party. We do not know if Nick left with them.

Our family sources told us … Michele had been anguishing to friends over the last few months that she and Rob were at their wits’ end over Nick’s mental illness and alleged substance abuse issues, and did not know what to do with their son Nick … saying, “We’ve tried everything.”

Billy Crystal and his wife Janice saw the slain bodies of Rob Reiner and his wife Michele … because TMZ has learned they arrived at the scene in a flash after a family member called them about the horrific discovery.

Sources with direct knowledge tell TMZ … Rob and Michele’s daughter Romy called Crystal and his wife not long after she called police to report finding her parents brutally murdered inside their Brentwood home Sunday.

Sources with direct knowledge tell us Billy and his wife raced over to the house — and were there so quickly, one source tells us — the couple actually saw their slain friends … and wanted to say their goodbyes.