I have a theory about this because it's happened to me several times and I used to be a package handler for UPS in my college days for a summer.

I think they scan it going onto the truck. It then slips between the cracks "literally" since the package is so small. They then just claim it's been delivered because they can't find it on the truck anymore and go about their day. We'd find small envelope packages all the time in between the gaps between the trucks and loading dock.