My 2 cents for what its worth.....I used to work at LGA and twice, I recovered Postal pouches from the ramp apron area along the taxiways. They had fallen off transfer carts (imagine a pioneer wagon train, about 5 carts hooked together) and both times the Postal Service had no idea where they were until I returned them. When a single piece of mail goes, I'd strongly suspect employee theft. When the pouch goes, who knows? Maybe they will turn up.
BTW, what does it mean when the flag is hanging at half-mast at your Post Office?? It means they are hiring........