Hello,
Taking approval on any leave ( Paid or Unpaid) is should be follow. The authorize person need to manage shift or work plan for team as well, and a sudden leave of an employee cause problem sometime. According to me, if leaves are planned then it should be informed to the manager or to the person who is authorize to approve or reject leaves. Leaves will be consider as Unpaid leaves or LOP if an employee doesn't have leaves in their account.