People leave various itms at the center of the labyrinth. I believe as symbols of a person (living or deceased) or of themselves.
"At the center of the labyrinth is a circular sculpture created by stonemason Dan Sieracki. The circle is a symbol of eternity and the fullness of life to which God calls each of us. This empty circle at the labyrinth’s center reminds us to empty ourselves so that we might be filled with Christ’s peace"