PA23 Flight Instructor reported rejecting a takeoff after a loud bang and buffet were encountered during the takeoff roll. After returning to parking it was discovered that an inspection panel had not been completely secured in anticipation of further maintenance action and the aircraft had been out of service.

Narrative

Shortly after liftoff; while accelerating in ground effect; we (the student/aircraft owner and I) heard a bang; followed by a continuous metallic slapping sound; accompanied by a buffet. We experienced the initial startle for a couple seconds; then made the decision to reject the takeoff; and land on the remaining runway. The landing was uneventful; with a normal taxi off at the end. We had made a reject call to the Tower; who then instructed the following aircraft (which I believe was a training flight operating in the pattern) to go around. Upon taxiing off the runway; we proceeded to taxi back to the hangar. Upon exiting the aircraft; we noticed a belly panel on the fuselage was not fully secured; and ended being bent and torn in spots. The student/owner called the mechanic; who then told him he had not completed securing the panel; as he did not think the plane was due to be flown for several days; and was waiting for water on the floor to dry. He assessed the damage as minimal and easy to repair. I had met the student on the ramp after the plane had been preflighted and taxied over. At that time I was unaware that any maintenance had been performed on the plane (and I had just examined the logbooks and verified everything was signed off just two days prior). The aircraft operated normally during taxi and run-up; giving no indication is was actually out of service. After this event; we discussed ways to alert pilots that the aircraft was out of service; such as a placard on the flight controls or engine controls.