How about a free flight model or an RC model out of range flying on it's own. I fly RC planks and rotaries and It's possible someone flying an RC plank had a radio problem or lost sight of the model and off it went. A well trimmed model could fly on it's own and if it had a tiny bit of rudder it could orbit and with a small amount of elevator and the C of G moving aft as the fuel got used up it could easily climb, with enough fuel a flight time of up to 30mins could easily be achieved.I've seen this happen and the model never found.
R