If the approval workflow is automated in the absence of variable elements, and a timesheet with no apparent overtime has not been automatically approved, please check the points below.
A timesheet with no overtime was not automatically approved
Check the settings of the approval workflow via Settings > Workflow. If the approval workflow only requires approval for deducted variable elements, and a timesheet without visible variable elements has not been automatically approved, it means that time has been allocated to non-visible time accounts.
Check via Settings > Time accounts if there are non visible time accounts:
Then check in Reporting > Accounts Report if hours have been allocated to these accounts for this timesheet.