I spoke to the carrier today, asking whether it's possible for something to fall between the seats, under something or another possibility in which it would remain on the truck. He told me it's impossible. At the end of the day, the driver has to go through the truck to verify that it's empty and that every package and flat (letter) is removed.
Although it doesn't make me happy, he suggested the possibility that the label got ripped and couldn't be scanned, but if that's the case, I'd expect a "RTS" (return to sender) stamp and that it would come back to me. The way I attach my labels though, it's highly unlikely that it would get ripped.
I'm going to remain optimistic (as you are) that the item will be delivered tomorrow as expected.
I haven't contacted the buyer but will reply if she contacts me.
(FWIW, another package I took to the post office and had scanned as "accepted" about 4 hours ago is still showing only as having the shipping label created. And I KNOW she scanned it in!)