Yeah Abby has used ASL before and has read lips in some episode , she and Gibbs used ASL a few times. I thought it was a cool scene too.That Abby and her brother Luca (NCIS) can sign ASL to each other, and that Jethro understands them. (I remember that Mark Harmon used ASL in one of his previous series, Reasonable Doubts, with Marlee Matlin.) A surprise scene in tonight's crossover episode.
I like Abby !