A choice of two coffins from WWII was used to decide which would rest in the Tomb of the Unknown Soldier in Arlington, VA. What happened to the one that wasn't chosen?

In 1958, the remains of two unknown soldiers (Unknowns) from World War II's European and Pacific Theaters were placed in identical caskets and on May 26 taken aboard the cruiser USS "Canberra" along with an Unknown from the Korean War.

The World War II unknown was randomly chosen by William R. Charette, an enlisted man and Medal of Honor recipient. The casket of the other WWII Unknown was buried at sea.

The Unknowns of WWII and the Korean War were interred at Arlington National Cemetery on May 30, 1958, alongside the Unknown from World War I.

https://wiki2.org/en/Tomb_of_the_Unknown_Soldier_(Arlington)+Newton

Given that, according to the Department of Defense site below, the remains of thousands of unknown soldiers are buried at Arlington, the remains that were not selected for the official tomb are probably buried there as well.

https://www.defense.gov/Multimedia/Experience/Tomb-of-the-Unknown-Soldier/