Arbiter's Grounds - Piece of Heart Locations
As with all the previous dungeons in Twilight Princess, Arbiter's Grounds is home to two heart pieces. The first one is extremely easy to find as it's located in the very first room, the same room you meet the four poes in. The second piece of heart is found after you get the Spinner in one of the largest final rooms of the dungeon with a ton of rails.
Both of these pieces of heart are relatively easy for you to find - in fact I would argue most of us get both of them on a regular play through from just simple exploration. You practically pass by each of these chests while progressing through the dungeon and both are in areas where you're left scratching your head wondering where you're supposed to go.
First Piece of Heart - Main Room
The first piece of heart available for you to acquire in the Arbiter's Grounds is found in the main room of the dungeon where you kill the first Poe and learn the Poe's scent for the dungeon. Off to both sides of the door that the Poes close you can find chests.
In one of the chests you'll find the Dungeon Map and in the other chest you'll find a Piece of Heart. If you're having trouble finding this location use my screen shots for guidance.
Second Piece of Heart - After you get Spinner
The second piece of heart for you to find in Arbiter's Grounds comes shortly after you get the Spinner. You'll find yourself in a giant room with a ton of rails all over the place - your objective is to jump over a fence and ride the rails to a new part of the room.
On your way to completing this objective you'll find a chest with this piece of heart in it. It's rather hard to miss.