With the USPS there is no Driver Release Areas, that is UPS. I work for USPS and if they are leaving signature items they are not doing their job correctly. The only thing that can be left without a signature is Express with waiver.
Have the USPS look up where it was scanned, the scanners have GPS and that narrows it down ask for the coordinates, if it was scanned in the post office as delivered it is considered falsifying information and can lead to the carrier being removed. Contact the postal inspectors to look into it. Someone is not doing their job correctly and it won't be fixed unless you report it. https://www.uspis.gov/report/
They have the tools to find out more about the missing package then you think.