The movie is Star Wars — Episode III: Revenge of the Sith. Upon the discovery of Palpatine’s dual identity of being Sith lord Darth Sidious, Jedi Master Mace Windu gathers a small group of Jedi to apprehend the Sith Lord. Sidious engages the Jedi in battle and is able to quickly kill all but Mace Windu. The Jedi master is able to go toe to toe with the Sith. Windu is able to render Sidious saber less and back Sidious up against the shattered window.
Anakin Skywalker enters the office to find Jedi Master Mace Windu pointing a lightsaber at Supreme Chancellor Palpatine. Palpatine immediately plays the victim. Sidious fires lightning at Windu, who is able to deflect the lightning back on Sidious. Windu tells Anakin that Palpatine is the traitor. Palpatine insists the Jedi are taking over and Anakin must choose. Skywalker is torn and against Mace’s pleas and wishes, and slices off Windus’s forearm. The arm and lightsaber begin the long fall to the city below. Windu follows shortly.
Sidious electrocutes Windu and tosses him to his assumed death. The rest is history.
What happens to Mace Windu’s lightsaber?
The lightsaber was assumed lost by many. However, according to the book The Last of the Jedi: A Tangled Web (via Wookieepedia), one year later, some Duros scavengers found a lightsaber and claimed it was Mace Windu’s and successfully sold the lightsaber to Senator Sano Sauro.
In his office, Senator Sano Sauro kept the broken lightsaber sold to him by Duros. The senator displayed the lightsaber proudly. A lost and last remnant of the keepers of the peace.
The Emperor would have been able to recognize the saber and say for sure if it was the same saber belonging to Windu or not. Other than that, many people would never know for sure if it was the same saber or a saber belonging to another Jedi. However, not many lightsabers were identical to the lightsaber wielded by Windu.