On December 20, 2006, approximately 1700 Pacific standard time, substantial damage to a Piper PA-28R-180, N4666J, was discovered while the aircraft was being refueled after the completion of a cross-country instructional flight at the Fresno Chandler Executive Airport (FCH), Fresno, California. The airplane is registered to Memley Aviation Inc., of Fresno, and was operated by a certificated flight instructor. The date, time, and circumstances, which resulted in the aircraft being damaged to the aircaft is unknown. In a telephone interview and in a written statement submitted to the NTSB investigator-in-charge (IIC), the flight instructor reported that on the day the damage was discovered he had undergone a checkout in the airplane, followed by a cross-country flight with a pilot-rated student. The pilot stated that prior to the checkout flight the check pilot told him that he had already done a preflight, but that the instructor could do one if he wished. The instructor reported that the did a "quick walk around" but did not notice damage to the landing gear or to the top of the wings. The instructor stated, "Had I done a full preflight myself I would have seen it." The instructor reported that after the checkout flight was completed he and his student flew to the Santa Monica Municipal airport (SMO), followed by a return flight to FCH. The instructor further reported that after landing at FCH and during refueling, the airplane's owner observed wrinkled skin on the top of both wings directly over the main landing gear, with wing rivets displace/popped out their secured riveted positions. The instructor stated that after being apprised of the damage, "...I knew that nothing that transpired during the course of my time in 66J could have come close to causing that kind of damage." Interviews conducted by Federal Aviation Administration personnel with several pilots who had recently flown the accident airplane failed to reveal who was responsible for the damage, or how the damage occurred.