A source tells E! News that the undercover couple was "all over each other" while attending a rooftop party at the Thompson Hotel.
And we're talking full on PDA here, Twi-hards, not just hand-holding. "They were kissing, nuzzling, holding hands and embracing," the source tells us.
"He's so into her," says the insider. "The way he looks at her is so passionate."
As for K.Stew? She seemed to be loving the attention from Rob. "She was so smiley," says our source.
More platonic partygoers included Taylor Lautner and the rest of the cast, who entered the bash through the back entrance. This is the second year in a row they've celebrated at the hotel.