After his grandfather's death, Angus goes to the park to talk to his grandpa's old friend. As they talk, you can see the chess pieces on the board, but a little later they've disappeared.
actually, didn't his grandpa's friend take them home because they were his? i mean, chess pieces would get stolen if they were left just sitting there in the park, anyone could come up and take them.
Just like books..movies have things called symbolism.
The pieces were gone because the old dude took them....and the King is still laying down to symbolize his grandpa dying.....it's along the same lines as whan Angus is the only one wearing red at the assembly and EVERYONE else is wearing blue....Angus is the Element that must overcome it's surroundings to mutate. Symbolism Look it up....