Rob Reiner's Son Nick Seemed Out of Place at Conan O'Brien's Christmas Party

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.