This is due to an employee who is assigned to this policy in the past or in the future.
To find employees assigned in the future, you can check this by going to Time > Time Off Settings > Policies > Policy Balances
This page contains details about the Policies and you can add the ‘future assigned people’ column to view the future assignees. Screenshot below showing this:
For past assignments, you'll want to use the Policy Assignment report 😁️ Head to Analytics > Reports > Time Off > Policy Assignments summary.
There you can filter by the change (Assign/unassign) and the policy types.