No idea, the most plausible explanation is that the driver stole the item and left the empty envelope. Although what they thought they could do with replacement

No idea, the most plausible explanation is that the driver stole the item and left the empty envelope. Although what they thought they could do with replacement foam pads for a particular set of headphones I don't know. But I'm sure it's possible to lockpick the door or do something else to the locker, as it was an outside 24/7 one. fairly unlikely though.