He hides the old man's body under one of the wooden planks/floor boards. However, he later goes insane and imagines that he can hear the heart still beating and digs the body out. It's been a while since I read it so I might be wrong, but I'm pretty sure I'm right.

Yeah mostly right so two police guys come over and his insanity begins to take over so he thinks that they KNOW he has the body under the floor (they don’t) so he completely loses it and tells them that they can stop pretending. He then tells them where the body is .

Explanation:

There are 2 unknown soldiers that are buried in the Unknown tombs, from World War 2 and Korea. Nobody knows for sure which 2 World War 2 soldiers are buried in the Unknown tombs in Arlington National Cemetery.

The Tomb of the Unknown Soldier or the Tomb of the Unknowns is a monument dedicated to deceased U.S. service members whose remains have not been identified. It is located in Arlington National Cemetery in Virginia, United States of America.

The World War I "Unknown" is a recipient of the Medal of Honor, the Victoria Cross, and several other foreign nations' highest service awards. The U.S. Unknowns who were interred are also recipients of the Medal of Honor, presented by U.S. Presidents who presided over their funerals.The monument has no officially designated name.

Explanation: On March 4, 1921, the United States Congress approved the burial of an unidentified American serviceman from World War I in the plaza of the new Memorial Amphitheater. On November 11, 1921, the unknown soldier brought back from France was interred below a three-level marble tomb.

The bottom two levels are six marble sections each and the top at least nine blocks with a rectangular opening in the center of each level through which the unknown remains were placed through the tomb and into the ground below. A stone, rather than marble, slab covers the rectangular opening.

Since 1921 the intent was to place a superstructure on top of the Tomb, but it was not until July 3, 1926, that Congress authorized the completion of the Tomb and the expenditure of $50,000 (with a completed cost of $48,000). A design competition was held and won by architect Lorimer Rich and sculptor Thomas Hudson Jones.

Quarrying the Yule marble (3.9 miles south of Marble, Colorado by the Vermont Marble Company) was a one-year process beginning in 1930. The cap was quarried on the first attempt but the base required three tries. The large middle block also required three tries. In late January 1931, the 56 ton middle block was lifted out of the quarry. The quarrying involved 75 men. When the block was separated from the mountain inside the quarry it weighed 124 tons. A wire saw was then brought into the quarry to cut the block down to 56 tons.

On February 3, the block reached the marble mill site (in the town of Marble) after a four-day trip from the quarry. Here it was crated, then shipped to Vermont on February 8. The block was sawn to final size in West Rutland, Vermont, and fabricated by craftsmen in Proctor, Vermont, before it was shipped by train to Arlington National Cemetery, Virginia.[8] By September, all 7 blocks were on the grounds of the Tomb site, at Arlington.

Assembly began in September 1931. An imperfection was found in the base, requiring three more quarryings. By the end of December 1931, the assembly was completed. Finishing work followed with the carvings on the die block by the Piccirilli Brothers under the direction of the sculptor Thomas Jones. (The brothers also carved the Lincoln statue for the Lincoln Memorial). The Tomb was completed without formal ceremony on April 9, 1932.

The Tomb  was placed at the head of the grave of the World War I Unknown. West of this grave are the crypts of Unknowns from World War II (south) and Korea (north). Between the two lies a crypt that once contained an Unknown from Vietnam (middle). His remains were positively identified in 1998 through DNA testing as First Lieutenant Michael Blassie, United States Air Force and were removed. Those three graves are marked with white marble slabs flush with the plaza.

The Tomb has a flat-faced form and is relieved at the corners and along the sides by neo-classical pilasters set into the surface with objects and inscription carved into the sides. The 1931 symbolism of the objects on the north, south and east sides changed over time.

There were almost 300 unknown tombs in Arlington National Cemetery. They include the following from World War 2 and Korea:

On August 3, 1956, President Dwight D. Eisenhower signed a bill to select and pay tribute to the Unknowns of World War II and the Korean War. The selection ceremonies and the interment of these Unknowns took place in 1958. The World War II Unknown was selected from remains exhumed from cemeteries in Europe, Africa, Hawaii, and the Philippines.

There are exactly 2 unknown tombs from World War 2, in the Arlington National Cemetery. Two Unknowns from World War II, one from the European Theater and one from the Pacific Theater, were placed in identical caskets and taken aboard the USS Canberra, a guided-missile cruiser resting off the Virginia Capes. Navy Hospital Corpsman 1st Class William R. Charette, then the U.S. Navy's only active-duty Medal of Honor recipient who was an enlisted man, selected the World War II Unknown. The remaining casket received a solemn burial at sea.