These days, everything is scanned at every point from check-in, truck load, truck unload, etc, with all movement carried out by GPS tracked vehicles. Yet, packages "disappear" from their system only to "reappear" a few days later. I have had this personally with all 3; UPS, FedEx, and USPS. It seems like with all the scanning, they would know immediately at which point the item went missing. But, for some reason, it doesn't seem like they do.
Just Friday, UPS was supposed to deliver a package from Amazon. I got a notification Fri. morning that it was on the truck and out for delivery by 7pm that day. At 6:49pm, I received the following notification:
"Unfortunately, UPS wasn't able to complete your delivery and needs additional information to try again. We're sorry for the inconvenience."
My translation of this notification is: "Sorry. The driver is tired and heading home. Hopefully, he will make it on Monday"