I believe that thorough investigation of all foreign object damage to civilian jet engines in flight will find that there are quite a few that are not caused by birds. Then I would look for records of military flights in the area. The dots will connect. It's not even necessary to use stealth jets to accomplish this. These aircraft fly set routes and there are many ways to suspend dangerous materials in the path of a jet engine at altitude. In WWII, the British used rockets towing cables with mines on the end of them to try to blow chunks off the wings of Nazi bombers. There were cables supporting barrage balloons that would rip a wing right off a low-flying aircraft. So yes, it is a legitimate inquiry.