Warning: spoilers ahead for Episode 8 of NCIS Season 18, called "True Believer."
News broke well before Season 18 premiered on CBS that NCIS would be saying goodbye to Maria Bello as Jack Sloane, but the question was of how the show would write her off. NCIS may not kill off every character that leaves, but even those who survive don't always leave the door wide open behind them. Jack and Gibbs headed to Afghanistan in "True Believer" for an incredibly dangerous mission to rescue a group of kidnapped schoolgirls, and the episode ended with Jack not coming back to the U.S. with Gibbs. So, what happened, and can she come back?
Understandably given her history, Jack was immediately invested in finding the kidnapped girls, even more so when she realized that a woman who had helped her during her time as a POW in Afghanistan was involved. The reveal that the girls were going to be sold to prevent them from going to school just raised the stakes. The good news is that Jack and Gibbs solved the case, rescued the girls, and were cleared to go home; the bad news for Gibbs was that Jack decided to stay in Afghanistan to help, and had never intended to move to a peaceful life in Costa Rica after all.
Jack explained her decision to stay in Afghanistan in an emotional speech to Gibbs, saying:
I have loved my time at NCIS. Been a really good path, but you don’t need me anymore….I don’t belong behind a desk. I never did. And you’re right. I was running. But I don’t want to do that anymore. I think I can make a difference here. I want to make a difference here. And I know you understand that. I’ll call everybody when I’m ready. And as for you, if you ever want to visit…
On the one hand, Jack staying in Afghanistan means that she's not exactly local to the NCIS team to drop by and see on a whim, and she left her fish with Torres for a reason. Jack isn't coming back for the foreseeable future, and Maria Bello is going to be busy. On the other hand, NCIS didn't kill Jack off, and the door is definitely open for her to return.
At the very least, maybe NCIS could show some phone call scenes between Jack and the agents she left behind in Washington D.C. Gibbs also cut off her suggestion that he visit her with a big kiss, but I think the invitation still stands! Maria Bello hasn't said anything bad about her time on the show, and evidently left due to work that is taking her elsewhere, not due to any bad blood.
That's not to say that NCIS brings back all the major characters that leave. While Abby is unlikely to ever return based on comments from actress Pauley Perrette, there are others who left alive and well without returning. Still, I think NCIS left more than one way for Jack to return, even if only for a guest appearance.