Key phrase is "could have been". My guess: Iran has been complaining of US surveilance flights over their territory, and have threated to shoot down any encroaching planes. Maybe an Iranian pilot got an itchy trigger finger, missed, and the stray missle headed down to the ground.
During the Iran Iraq war Iranian pilots who fired missiles in combat and didn't get a kill were likely to be executed. Consequently, there wasn't much air to air combat going on. If an Iranian fighter jock popped a missile at a drone and missed his ass belongs to Allah!
Wouldn't it be funny if this did happen, and the stray missle happened to hit the nuke plant? DOH!