I happened to come across concept art that reveals Bolivar Trask, the inventor of the Sentinels who was played by Game of Thrones' Peter Dinklage in X-Men: Days of Future Past (2014), was supposed to appear in the 2016 sequel, X-Men: Apocalypse.
At the end of Days of Future Past, after Mystique saved President Richard Nixon, the Sentinel program was cancelled, preventing the dark future in which advanced Sentinels hunted down and exterminated mutants with ease, and Trask was arrested for selling his weapons to enemies of the United States of America.
In a mid-credits scene, which was included in the Rogue Cut, it's revealed that Trask was being kept in a prison cell beneath The Pentagon, which was the very same one Magneto was confined to earlier in the film for assassinating President John F. Kennedy.
With that in mind, it makes sense that the Trask concept art, illustrated by Bartol Rendulic, is for an ankle bracelet. That would imply that he would have been under house arrest or on parole in X-Men: Apocalypse. Perhaps, the U.S. government would have sought his help when Apocalypse started destroying the Earth, thus reigniting the Sentinel program.
Following the critically acclaimed global smash hit X-Men: Days of Future Past, director Bryan Singer returns with X-MEN: APOCALYPSE. Since the dawn of civilization, he was worshipped as a god. Apocalypse, the first and most powerful mutant from Marvel’s X-Men universe, amassed the powers of many other mutants, becoming immortal and invincible. Upon awakening after thousands of years, he is disillusioned with the world as he finds it and recruits a team of powerful mutants, including a disheartened Magneto (Michael Fassbender), to cleanse mankind and create a new world order, over which he will reign. As the fate of the Earth hangs in the balance, Raven (Jennifer Lawrence) with the help of Professor X (James McAvoy) must lead a team of young X-Men to stop their greatest nemesis and save mankind from complete destruction.
X-Men: Apocalypse will arrive on Digital HD on September 9 and on Blu-ray, 4K Ultra HD, and DVD on October 4.