I want to terminate a HRA plan. There are no outstanding reimbursements to be paid. Any other steps besides notices that must be done?

The termination should be adopted by the corporation, a timetable established for shutdown and notices provided to participants. Then the plan can be closed down. The timetable should include a final date for incurring claims and a final date for submitting them, with at least a 90 day lag.
How can you know about unpaid reimbursements? Claims may well come in of which you are currently unaware.