Chicago P.D. will be missing one of its original characters when it returns for Season 7, but you wouldn't have known it based on how Season 6 ended in the spring. Although news had already broken that Jon Seda, who had played Antonio from way back in Chicago Fire Season 1 before P.D. even existed, would be leaving the series, the finale didn't give him a sendoff that would explain why he wasn't around when the show returned. Now, Chicago P.D. showrunner Rick Eid has revealed that P.D. isn't finished with Antonio Dawson, despite Seda's absence.
Antonio will be missing when Chicago P.D. picks up in Season 7, but his absence won't remain a mystery forever. Rick Eid revealed in comments to TVLine that the Intelligence Unit "will ultimately figure out what’s going on with him." So, even if Jon Seda isn't being brought back for a premiere sendoff like Chicago Med is bringing back Colin Donnell, his story will be revisited. When last seen, Antonio was in his car, getting ready to take a pill.
The ambiguous ending could have meant many things for Chicago P.D. Season 7 that wouldn't necessarily have required P.D. to make a big deal about Antonio's absence. The pill could have meant a relapse for Antonio, and that could have led to his tragic (off-screen) death due to an overdose or deal gone wrong or just spending time in the wrong places around the wrong kind of people.
Alternately, it could have been a sign that Antonio couldn't stick with the CPD and would leave over hiatus, with his departure simply being mentioned in Season 7 before the rest of the unit moves on. Rick Eid's statement that the Intelligence Unit will "ultimately" figure out "what's going on" could mean two things: the story isn't resolved right away and Antonio isn't dead. Something can't really be "going on" for a dead man, right?
Could Antonio have quit because the job wasn't good for him, or because he wasn't good for his coworkers anymore? Could he have been sent on an undercover mission of some sort? Did he join Erin Lindsay in her new job, after she too left off-screen between seasons? Is he hanging out with his sister in Puerto Rico? We won't know until Chicago P.D. returns later this month, but it sounds like Antonio fans can count on some resolution that honors somebody who was such a huge part of the whole Chicago universe.
Of course, the Intelligence Unit will have more going on than just a missing member of the team. Season 6 ended with the reveal that the newly elected mayor had been murdered, and none other than Voight had a major motive for killing him. Halstead and Upton were facing potential moves within CPD, with or without each other, while Ruzek has stuck in jail due to his decision not to turn on Antonio. Throw in the gritty new cop who is joining the series and the complex new crime lord, and there will be plenty going on.