Substantial structural damage was found during a maintenance inspection. The date, time, and location where the damage occurred remains unknown. The pilot and any other occupants who may have been on board at the time are unknown, and it is presumed that there were no injuries. During an inspection, the airplane's mechanic found that the firewall was structurally damaged. The operator stated that she does not know when, where, or who damaged the aircraft. She further stated that there were three individuals who flew the airplane after its most recent 100-hour inspection. All the pilots and their previous passengers stated that their past flights and respective landings were uneventful; there were no witnesses to state differently. The operator added that the damage done to the airplane was not externally visible and could not have been detected during a preflight inspection. The date, time, and location utilized for this accident is referenced from the last flight for computer entry purposes only. A determination was not made regarding the identity of the pilot(s) that caused the damage to the training airplane.