Missile misses balloon. Yeah right. Anon's supposition is that somehow, somewhere, right now sits an unused sidewinder missile. And the extra missile that missed was never fired. Instead that serial number goes off the books and dark. In the hands of unknown parties.
Alternately it could have been an accounting for a serial number of a sidewinder that was previously fired at target unknown. (previously expended off the books) and this is a way off making physical inventory match records. Just a hunch, but something just seems off about a sidewinder missing a target like that.