"It must've been difficult to write about a guy who gives frustratingly mixed signals about who they're attracted to, huh, Dan?"
"...What did you do?"
THAT WAS YOUNGER ME! HE DOESN'T COUNT!
Annnnyway, I don't think a friendly hand wave (while presumably saying goodnight) is a bad way to say goodnight. It's just very, very platonic, and Sarah is trying to figure out how Sam feels.
Also worth noting is that Grace is biased about hugs, and will naturally assume there is must be some very specific and special reason if someone doesn't want to hug (an acceptable reason being "I'm Susan").