It is neither explicitly planned, nor predefined, but the European Commission may, at any time during the implementation of the project and up to five years after the end of the project, ask for financial audits or technical audit/review to be carried out, by external auditors/ experts, or by the Commission services themselves including OLAF. You need to keep your contractual documents and documentation of results elaborated at the defined milestones and specifically those main milestones that served for the review of being able to decide on the reimbursement of costs.