Actually, while most post offices do not scan the package when it goes on to the carrier’s vehicle I have seen it done. I ship books for a living and routinely check the delivery confirmation status, and have sometimes seen a scan that indicates that the “Item is on truck for delivery.”
There was one PO up in Ohio one time that showed every move the package made, as it arrived at the PO, as it was put on the carrier’s vehicle, as it went to the label address and then went back on the vehicle, was forwarded to the PO of the forwarding address on file, was put on the next carrier’s vehicle, was refused at the forwarding address and returned to the second PO, was put on yet another carrier’s vehicle, and then sent on to a third address on file at PO number two.
At the same time I have seen situations where an item has indeed been delivered but the carrier failed to scan it, or an item shows no scans at all, not even an acceptance scan, until it is scanned as delivered.
Bear in mind that, as you very well know, the PO is a human endeavor, with all the variety that results from people doing the job as they were taught by whoever taught them.