NCIS explains how Ziva David is alive in the season 17 premiere, as well as how she's remained hidden for all the years. Cote de Pablo reprised her iconic role as Ziva in the NCIS season 16 finale, "Daughters", but it wasn't confirmed at the time if she was a hallucination or really in front of Gibbs, in the flesh. That much was confirmed prior to the season 17 premiere when an NCIS trailer revealed why Ziva had returned after spending the past few years under the radar.
As it turns out, Ziva was being hunted by a woman named Sahar, and that person was now coming after Gibbs. So in order to protect Gibbs and to finally reunite with her daughter, Ziva resurfaced in the episode "Daughters" and is now working alongside her former boss once again. Ziva's return has been the main talking point regarding the flagship NCIS series ever since the season 16 finale aired, and now, thanks to the NCIS season 17 premiere, "Out of the Darkness", details regarding Ziva's return and where she's been have been unveiled.
There were a lot of questions going into the NCIS season 17 premiere, and the writers didn't waste time giving viewers answers. After McGee and the team comb through Gibbs' home, Bishop heads to Ziva's old office, where a lady named Odette tells her: Ziva knew she was the target of the farmhouse explosion in the season 13 episode "Family First", so she faked her own death and went underground to protect her family.
After Odette explains what happened to Ziva after season 13, the question that immediately comes up is, does Tony know Ziva is alive? Unfortunately, it seems that viewers will have to wait quite some time to get that answer, as Odette refuses to say yes or no and Ziva is preoccupied with hunting down the person hunting her. Later in "Out of the Darkness", Ziva comes face to face with Sahar, but whether or not she truly gets her revenge and prevents her family from being put in harm's way remains to be seen.
Bringing Ziva back after all this time makes sense, given her story. It could've taken her years to uncover the identity of the person who wanted her dead, but how it all resolves is something that will be at the forefront of NCIS season 17, as Cote de Pablo is scheduled to appear in a handful of episodes at the start and towards the end of the season.