Night terrors. She's probably not awake enough to realize you're there. It can take some effort to wake ds enough that he knows I'm there. But we always go in. Once he's aware enough to calm down he goes back to sleep fast.
Ditto night terrors. DS (10 months) gets them too. I stick the binky in, and he goes back to sleep. I assume at 18 months she's done with the paci. In that case I would not leave her alone, she could hurt herself if it's exorcist-type activity. Try undressing her to wake her, turn on the lights, whatever it takes to wake her.
Previous posters advice to wake the baby up is wrong. It is easier for one to fall back to sleep than to wake up when having a night terror (and that's the endgame anyway). Go in and blandly soothe and ensure safety. Lay the kid back down, gently rub his or her back, etc, but do not make any effort to wake the baby back up.